what are some things at the begining of the game that i should do to have a successful tank

There are a lot of different strategies for successfully playing Fish Tycoon. This thread contains several that you might find helpful.

First, find out which one of your fishes is worth the most. The one I used was an 'Orange Fruitfish', but lots of the Fruitfish are worth over $10. Then breed this fish over and over again, and then sell the doubles to make money. Once you have made a lot of money, research the Enviroment, as this really helps when it comes to making and breeding rare fish.