As previously stated, you should have a mix of revolving (credit cards) and installment (bank loan, car note) credit.
If an account is in collections, unfortunately paying it off will do nothing for your score.
If you have one late payment with a creditor and have been on time ever since, you could write a "goodwill" letter to see if they'll delete the late payment. If not, the only thing that helps is time.
Pay your bills on time! A single late payment will destroy your score.
Any charge offs, collections, etc in bad standing can get removed after 7 years (usually automatically)
When I bought my first car in 2007 I was in the 400s
so imagine my interest rate.
Now I'm at 787 (Experian, you'll have a different score for each bureau)... can't seem to break 800 (aiming to do it by the end of the year).