To me, a perfect startup tank is this ...
20 gallon tank (long or standard)
1-2 hydor koralia nanos 240 gph
Lights (depends on fowlr
Pair of clowns
CUC (can include starfish. I recommend sand sifting starfish)
This is a basic setup that will be great for a beginner. Get a decent t5 or led
setup and you can start adding coral. Don't worry about breeding as this will happen on it's own naturally If the clownfish decided to mate.
Try not to over complicate it. A pair of clowns is simple to keep!
As for mixing salt, there's different ways of doing it. For stating out, I'd fill the tank with RO
water and then turn on filter and power heads and start adding salt according to directions!
Hope this helps