Dummies' Guide To UNIX "find" command:

find . -name "*.txt"

Tough, ain't it? Recurse from home dir on down for all files ending in .txt. Go.

[Update: It's been pointed out to me by a non-*NIX dummy -- why they're reading the Dummies' Guide to UNIX "find", I have no idea -- that the above call is likely case sensitive. He also provided a case insensitive alternative, which is debatably more useful and more dummy-friendly. You'll have to figure that out on your own, though it might be...
find . -iname "*.txt"]