Dominant 7th chords have a cool sound and are used in many styles. The names are abbreviated to "7" so you never need to write out Dominant 7.

These are all covered in Stage 5 of my Beginner's Course, so best learning them there, and you'll also get context because we learn how to use them in Blues!

The Open Chords for beginners include B7 (of which there is no regular easy B chord!) and there are a couple of different suggested fingerings for both E7 and A7 - use whichever is easiest for you - or use the one that sounds best to your ear if you can play both easily!