Like major and minor chords, 7th chords can be built on any of the 12 notes used in music. By memorizing the formula for a 7th chord (1 3 5 b7) and choosing a root note to apply the formula to, you can create C7, D7, F7, Bb7 or any other 7th chord. An easy way to learn 7th chords is to notice that a 7th chord is simply a major chord with a flattened 7th note added to it.
All 7th chords have four possible inversions, which gives you a variety of positions and fingerings for playing them.