What is the best way to improve credit score if I have no credit?

am debt free and have been since my bankruptcy over 12 years ago. I recently looked into getting a mortgage, but was told I can't get one because my credit score is too low. I went on freecreditreport.com and got a copy and there are some things on there that are not identified, and I don’t know what they are. So, how do I have those removed? I am assuming that will improve my credit score so I can get a mortgage. Advice is appreciated!

How do I improve my credit score?

just checked my credit score – it is (embarrassingly) in the high 500s. Yes, it sucks.

There’s nothing really terrible on my credit report – no bankruptcy, no collections. I was late 30-60 days a few times, but that was back in 2003. Everything has been current and paid on time since then. I believe the thing that is lowering my score is my credit usage/utilization – almost 100%. I have about 5K in CC debt on 3 CCs.

Now that I can afford to pay this off slowly, would it be better for me to pay off one CC at a time or a little off each one, gradually paying all 3 off? Does it make a difference?

I don't think there’s really much more I can do to improve my credit score except to pay off this balance and increase my available credit. Would it help to get a secured card with a high credit limit (which would increase my available credit)???

Help me please.