Hi!
Guppies, like many Central American livebearers tend to prefer neutral to hard/alkaline water, but are less fussy about this than mollies, say. It is important to maintain good water quality (regular water changes) as theyc an be prone to finrot and other bacterial infection. They are no longer considered hardy enough to cycle a tank, so I would advise you to do a fishless cycle unless you have a mature tank and are able to use its filter media.
Your tank is about 34 US gallons, so you could get a good shoal of male guppies in there (they grow to about an inch). Which is good, because this makes aggression less of an issue- and it should look spectacular. The only problem might be that guppies don't live terribly long 18 mths-2 years perhaps), so you might find yourself having to stock up again at regular intervals, if you are not breeding your own. But they are absolutely lovely fish. Though I personally like the females best.
When you buy them, look for males with strong hardy bodies, who seem to be swimming without difficulties. Inspect the tank very carefully- if any of its inmates seem to be sick or fungused or even dead- don't buy from that tank!
Guppies thrive on a good diet of quality flakes, with live food maybe once a week and veggies (such as peas, sprouts, spinach) once or twice a week. They'll appreciate a few plants or some other feature of interest in their tank: being lively and inquisitive fish, they don't want to get bored.