Hi Andrei,
You can google "nitrogen cycle" and see more in detail but you need some ammonia (fish/food waste) in order to kick-start the 'bacteria colonization' process, hence either feeding the tank with food without any fish, or adding ammonia directly or putting some fish in the water...
Your pH of 7.6 is fine for the likes of the Danio Rerios for example. Just make sure you acclimatize them over a small period of time from the shop water to your tank water.
To begin with, I would recommend you get the fish that match the pH of your water rather than trying to change the pH to match the fish you want. It can become complicated quite quickly !
I personally find guppies cool. It's funny to see them giving birth and growing on. Don't let the fact that they reproduce relatively quickly stop you. It's all down to a question of taste/preference.
Regards,
Valerie