Don't max out your credit cards. It is best to hold credit card balances below 30% of their limit.
For someone starting out and trying to build their credit, they need to maintain accounts for a long period of time.
If you pay off collections, be sure and ask for a letter stating the account is paid in full.
For a home loan, we need proof that it is paid and items get sent to your credit report when placed for collection. Unfortunately, many of them do not follow-through and report to the Credit Bureau when it has been paid.