Ultimately, what a person needs to improve their FICO scores and build credit are three open credit cards (secured or unsecured) in good standing and one open installment loan in good standing such as a car, home, student, personal, share secured, or credit building loan. This combination is what the myFICO score theorists here have determined is what you need for optimal credit building and FICO score. You can have more CCs and more installment loans, however, this will not increase your FICO scores.
Next, is paying in full all of the credit card balances each month, before the posting date, except one. This is called the All Zero Except One (AZEO) method. The one credit card you allow to post a balance needs to be less than 8.9% of the credit limit of the card. So using one card each month to buy lunch, letting it report and then paying in full will maximize FICO scoring.
The installment loan will have its greatest impact on your FICO score when the amount owed is at its smallest such as a few months before the loan is paid in full. If you don't have an installment loan you can check into SelfLender or a Share Secured Loan at a Credit Union.
Keep in mind, building credit is a marathon, not a sprint. It involves demonstrating to a potential creditor that a person can handle credit responsibly. Having open, active accounts that are being paid on time and low credit utilization is the fastest way to build good, solid credit.