What is the best way to improve my credit?

I made a mistake concerning my student loans a while ago and have a negative account (it is closed now). I am 22. I don’t need a lecture on the stupidity of this, I was 18 and naive.

A few questions:

1. Will it be impossible for me to get an apartment?

2. I know such things take a long time to improve but I can’t find any trustworthy sources. Do you have any advice? (I know pay my bills on time, I will have a new set of student loans to pay off starting in December)

3. I have an unused credit card, should I close it?