Hi, I would start with at least a 40 gallon as with a twenty five you will only be able to keep about 3 fish that grow to be up to about in the 3 inch size safely. You don't have to start with a fresh water tank, but they are a lot easier. If you want to start with saltwater, do yourself a favor and buy and read at least one good book on the subjuct before you buy anything. This way you know what is involved and get a better idea if it is something that you wish to do. A couple of good books are "The Conscientious Marine Aquarist" By Robert M. Fenner, also, "The Book of the Marine Aquarium" by Nick Dakin is a great book. "The Complete Book of the Marine Aquarium" by Vincent B. Hargreaves is also a good book. Any of these, or a couple of these should give you an idea of what it takes to be successful in this fun but challenging hobby. Please don't be like a lot of people on this board who buy first, kill all their first fish, then learn later! Start off right by doing your homework to avoid costly mistakes later on.
A lot of good books are for sale on this site, or other online book sites like Amazon.com. Good luck, Lesley