Welcome to the boards! Everyone is already giving you really great advice to build a strong credit portfolio.
I just wanted to encourage you to be patient. It could be really difficult because some days just feel hopeless. Some days you're gonna feel like you're never gonna hit your goal score or get that credit card you really want. I encourage you to be patient. The best tool for fixing credit is time. Time heals a lot. So, allow the time to pass while you work to send out goodwill letters for the accounts you have already paid and send PFD letters for the accounts that are still in collections.
After reading A LOT of discussion on here I have learned the secret sauce is:
1. GW paid collections to have them removed
2. PFD for the accounts that are still in collections
3. Get 3 bank credit cards
4. Get an installment loan (home, student, auto, or private loan)
5. Pay all cards in full every month except for 1 card - keep that card at >9%
6. Never ever ever be late with your credit card payments
if you do these things for 12 months, you will see improvement. But be patient and don't give up...the ONLY thing keeping you from your goal score or your goal card is if you stop trying.
The patient ones win this game 
Good luck!