I am getting a guinea pig and i need some advise?

I am getting a guinea pig in august for my birthday. Its only about 6 weeks away. I have done about 2 weeks of research and have finally gotten my mom to say yes. So i havent gotten anything for it yet. So should i get everything for it first set it up then get my guinea pig. and how am i supposed to know how much to feed it. I know the people at petco will tell me but i want to get my piggie into a routine. How do i do this. Please help. And tell me somethings your guinea pigs like if you have one. Thanks