First get your reports. Either through here:
or from a monitoring service like USAA. It's free to join USAA then 12.95 per month for all three reports (not scores, there's are FACO's) and you can check every day and get alerts via email or text when stuff changes. This way you'll know exactly what your working with. Don't guess the security questions. If you don't know don't find the answer cause they'll not give you your reports if you get them wrong. I highly recommend the USAA option.
For scores either buy your score here or sign up for the scorewatch trial for 10 days for free. These are the real scores. Everything else is crap.
If your file is just thin then you can get to gardening. App for a sub prime starter card or two with a low limit. Search hear and read for opinions. If you can't get a starter card then do the secured card thing. You'll have to pay to get these cards and they'lll have low limits but it'll grow your pretty fast. Charge a couple of small things on this card or maybe a monthly bill and don't use it. Just garden it. Watch your credit reports and when you've had it for 3 months or so and it's on your report and your score is up app for a real card or two. I went with chase. App in rounds then don't app for ANYTHING that's going to be a hard pull for months while your cards report and your average age of account gets longer
and your score goes up.
Now if you've got some bad stuff on your report then you'll need to do what you can to get that cleaned up. Some stuff will drop off after 7 years from charge off date. Any inaccuracies or small unpaid balances can be dealt with by settling with assurance it will be reported and for inaccuracies contact the credit bureaus or creditors.
I went from one single small closed line on my report from 12 years ago to having a 740 score, 3 cards and an AMEX with a 6k limit in a little over a year! It's a game just gotta learn the rules and strategies. 4 months is not a lot of time though.
One thing I did that has really helped me build credit fast is get an AMEX Blue Cash card as my second real card and as soon as I was able to apply for a credit limit increase after day 61 from activation I did via their website. You just hit a button and request up to 4x the limit and it's just a soft pull. I got bumped to 6k instantly and it dropped my utilization down to like 7 percent and my available overall credit to almost 8k. I'm not goint to request credit limit increases from my other cards for a while becasue they often are hard pulls that effect your credit score. AMEX is a great company to have an accoutn with cause they have great benefits and do things like that that other companies won't usually do.