In whatever software you choose, you should always start with yourself, and go back through time with your family. Find a software program that fits your needs the best, either maintained on a website online or solely used on your home computer. The genealogy software for use on your computer can be found for free by downloading from the internet, or you can purchase the software packages either using internet downloads or CD's. Some examples of software are noted below.
Enter the data that you have gathered and make notes like "Per Aunt Sally, Jan. 2013" or "Per letter in possession of Mark Smith, Sep. 2012". Make notes on conflicting information, as well, like "Possible death date of 1924 is per Aunt Sally; death date of 1926 was per conversation with mom in 1983".
Track whole family groups through time using census records to verify (or narrow down) those birth years and birth locations, as well as, to see if other relatives were living with your direct line at some point. This is part of doing Cluster Genealogy. By keeping your research to just your direct line, you are in essence putting blinders on. Families quite often stayed together, and by tracking that sibling or cousin, you may find clues as to where the family lived overseas.
Whenever possible, it is critical for you to be able to see the original documents or records yourself. You can not trust a transcriber's interpretation of the spelling of your family name, or the numbers in their ages. For example a transcriber might have interpreted a last name as "Rose" when it was actually "Ross", or interpreted an age as "54" when it was actually "59", or they may have left out the birth state or country that was shown in the original image.
While your goal should be to document your sources, and have copies of as many records as you can find for your family, keep in mind that there are various degrees of accuracy. Every document or record created is only as good as: the information given to the document creator at that moment in history; the information written correctly (meaning that the person speaking said one thing and the person writing it down wrote another thing); the information that was transcribed by others. We are all human and we all make mistakes. Sometimes our ancestors lied on purpose, for example, either to get into America, or to get into a world war because they were afraid of rejection. Also keep in mind that the earlier in a person's life that a document was created, the more likely it is to be the most accurate.
Another area to find clues on your grandpa is online family trees, found at various websites. However, I suggest never trust the other trees without first trying to prove that the information in those trees is accurate. I use other online trees for clues, but I have to prove the data somehow, and will often contact the tree creator and ask questions on how they came up with their information. People who are doing their own genealogy research will often give their distant relatives all the data that they have found.
Sometimes there literally are no records any more because of fires, floods or wars. If records are not available online or have not been microfilmed, you might not have the means to travel to the cities or towns to view them on site.
Sometimes it is as simple as looking in the wrong place. I once helped a man, who I met by chance, who spent weeks looking in County Cork, Ireland, for his grandmother to no avail. After about 10 minutes of looking, I found a 1910 census record for the family. It showed she was born in Wisconsin, but her parents were born in Ireland. He didn't realize the importance of a census record and how it can help find information on the family. Another example is, if your grandpa happened to move in with one of his granddaughters who lived in Nebraska, after his wife passed away in Wisconsin where they lived for many years. You've got to search where the granddaughter was living and not just search in Wisconsin for him. Follow the whole family and not just your direct lines - at least 2 generations down from a couple.
Death record information is often supplied by relatives under emotional duress or in-laws who might not know where grandpa was born exactly, or who his parents were, or might have only known that his mother was "Mary". This misinformation is carried into the newspaper obituaries and onto grave markers.
If your ancestors lied about their ages, you might have a hard time finding them. For example my great grandmother lied about her age because she was 5 years older than her husband. Another example is my great grandfather lied about his age to get into the NYPD back in 1880 because he was too young. These relatives carried that lie to their graves, in fear of being found out and reprimanded somehow by their employers, friends, and family I suppose. The most common area of age issues, besides incorrect census records, is the WWI draft registration cards. I found that most men who have conflicting birth years have changed it so that they could get into the war in the first wave of registrations.
Using all the various documents and records available for your grandpa, and viewing them all as a whole package, is the only way to put his life together as a complete unit. One census record from 1910 is not enough evidence to prove that he arrived in America in 1876, but 2 census records and an obituary might do the trick. Only then can you really start hunting for him in other countries prior to 1876.
You want to make sure that the direct link between you and your grandpa is proven. Review how you got your information, and determine its accuracy. For example, did you get his birth year from one census record or was it from someone else's online tree? Try to start from scratch with him again. Coming back to a person, after a period of time has elapsed, will sometimes give you a fresh look and you will have new ideas on where to search. A time delay might also allow another database to be uploaded to the internet.
If you have an Ancestry.com family tree, there are two areas that cause big issues: Clones and "Living" people. Periodically go through your listing of all names, to verify that you don't have any clones (duplicate people sometimes created in your tree, by merging other family trees to your tree, or by merging records to a family group.) If you have clones, you need to be careful on which person to delete. You don't want links to their parents and siblings lost, and you don't want to loose any records that you've already attached to the clone. You'll need to verify which person is the clone, and which person is the real McCoy. I can offer ideas on the best way to clean your tree if you spot clones.
Also pay attention to the people in your tree that are still living. Ancestry.com does not automatically default a person to "living" status (preventing the whole world from seeing their full names, birth dates and birth locations). Review your tree to make sure they are marked as "living", by seeing that word in the "death" field on a person's profile page. If you need to correct it, click the "edit this person" tab and select the correct box, then save your changes.
A series of videos showing how to attach records to people in your tree on Ancestry.com.
Part 1 - when to use "alternate" options;
Part 2 - choosing uniform formats for names, dates, and locations;
Part 3 - issues that can arise when attaching records to whole families at the same time.
Find A Grave.com
US Gen Web.org
National Archives.ie - Census Database
Ask About Ireland.ie - Griffith Valuation Index
Ireland Reaching Out.com
If you are interested in a particular country, and need some ideas on websites available, please email me. I will gladly poke around to see what's out there, and email you the links that I find. There will be no charge for this, since it truly will only take a few minutes of my time. The whole point of this website is to help people find their grandpa!