How to build credit?

I have a secured credit card and I use less than 10% of it. I pay it off on time. I'm really young. When I lived at my parents & I JUST turned 18 so when I went to the doctor, that bill went under me. Even though I was still practically a child. I didn't find out till a few months ago and I'm now 23. It's $700. I don't know much about credit so I'm assuming that's keeping my score down. It's 596. I got the credit card a few months ago and it's gone up by 100 points. Is there anything I can do to build it more? I've never done anything crazy. I just have that bill I found out about when I signed up on credit karma and the credit card.