Have you looked at the links that have been posted?
What we're trying to say is; you don't add any fish until you've cycled your tank; 'cycling' means growing a colony of good bacteria in your filter to eat the fish's wastes for you before you add any fish.
We never recommend fish in cycles, as exposure to the toxins that are present in a tank that is not cycled can lead to long term health problems for the fish, or even kill them.
Apart from that, we couldn't recommend any fish species for you, as you haven't told us how big the tank is, whether it's heated or filtered, nor how hard or soft your local water is. We'd need to know all those things before we could suggest fish that would be happy in your set up
SCG, can I suggest you read the post by Fluttermoth above. You don't know enough about Sharone's tank to make that recommendation - she may not have a heater for a start, so any tropical fish would be out. Furthermore, in my experience, bettas do not thrive in large tanks - being such a sedate fish, if there is too much going on around them, it can freak them out. I would not have a betta in a tank larger than a 100-120litre, and you yourself asked what size the tank is.
I appreciate that you are trying to help, but you have to tailor your advice to the particular circumstances of the person asking for the advice.