I've tried a few software programs for building family trees online. Each has it's pluses and minuses.
My favorite by far is MyHeritage.com. They offer free software you can download and then link up to their website. The great thing about them is they make suggestions of matching people in their database. I've found several missing family members this way. Now, I have also found mistakes, but that is why you have to verify and quantify your info.
Another great program is Ancestry.com. You have to pay for this access, but I find it well worth the price. You can research census records, military service, even old yearbooks! AND, you can directly save the information you find to the person in your tree it is for.
There are many other great programs for building your family tree. Check them out, see what has what you want to track for your family and which format you prefer. Ask people what they use and why.
But most importantly, start that tree!