What is a "Garmin 650"? Oregon 650? Montana 650? Rino 650? It probably doesn't make much difference in the context of your question, but since you have asked "where do I start", when seeking help in the forums it makes things easier if you provide as much info as possible.

How will you been using your GPS? What do you expect from it? Do you need a lot of "hand holding" with technical things? If so, then Garmin provides full tech support when you purchase their maps and can talk you through any problems you have with installation. Their maps also have some features not available on any of the free maps on this site. See this discussion:
http://forums.gpsfiledepot.com/index.php?topic=4036.0There are some excellent free maps here as well. But you will have to do a little more work to get them installed in some cases. The problems are almost always solvable, but the process can be very frustrating for some non-technical people.
I'd suggest just browsing the maps here and downloading anything that looks interesting - it's free so you don't stand anything to lose.
http://www.gpsfiledepot.com/maps/This tutorial explains the installation process:
http://www.gpsfiledepot.com/tutorials/how-to-open-maps-in-garmin-basecamp/And this tutorial will tell you how to send the maps to your GPS:
http://www.gpsfiledepot.com/tutorials/how-to-load-maps-on-my-garmin-gps-unit/Hopefully you'll find some that you like and then you'll be all set. If you have problems you can get help here in the forums or you can contact the author directly. But if you find the whole process frustrating, then you might be a candidate for purchasing Garmin's commercial products.
If you want aerial imagery, Garmin Birdseye is a pretty good deal at $30 for 12 months of unlimited downloads (you can keep what you download even after the subscription expires).