Credit cards?

Zerenity • 30. 🌵 Arizona Living
Okay, so I'm posting again since I can't find my topic of where I got my answers last. 
My credit isn't worth anything. And I've been suggested to set up a secured credit card, and make small payments to build my credit. I have one selected, however do I get to decide how much I put down, or does the credit card company do that for me? How long will it take for my credit to increase after I've made a couple payments? This is for a $200 credit line. Nothing too big. 
Any tips so I don't go over my 30% credit usage or whatever that is called? 
Thanks ladies.(: