Only get male guppies, they look nicer and if you get females they'll breed like anything and you'll be over run with the little critters
It's a good idea as well to work out the amount of fish you can keep in your size tank, i think its about 1cm of fish for every couple of litres. Also check what size your chosen fish will grow to. We had 2 plecs which we had to rehome as they grew to well over a foot each
