1) First you need to cycle your tank, in our advice for beginners section is a guide to tank cycling.
2) With guppies, unless you have somewhere to give the fry to or a way of culling them you will want to keep a same sex tank (if you have a mixed sex tank 1:2 or 1:3 male to female ration will keep the fish happiest)
3) We would need to know the dimensions of the tank to give the best estimate of how many of those you could keep but rough figures.
All male guppies 12-15 at most
All neon tetras 28-30 at most
All Black Skirt Tetras 18-20 at most
For a mixed species tank with Guppies and Rummy nose Tetras
6 x Guppy
10 x Tetra
