First of all, if you fly with one particular airline, consider becoming a member of their frequent flyer program. The loyalty programs are usually free and they allow earning miles on flights.
As for a credit card, you can apply for a credit card with rewards program. Most cards will allow you to redeem rewards for cash which you can use for anything you want. The cards will also allow you to improve your credit, provided you keep all your credit accounts in good standing, and later you will be able to qualify for an airline credit card like the one offered by United Airlines.
Disclaimer: This editorial content is not provided or commissioned by the credit card issuer(s). Opinions expressed here are the author's alone, not those of the credit card issuer(s), and have not been reviewed, approved or otherwise endorsed by the credit card issuer(s). Reasonable efforts are made to present accurate information, however all information is presented without warranty. Consult a card's issuing bank for the terms & conditions.
All rates and fees, and other terms and conditions of the products mentioned in this article/post are actual as of the last update date but are subject to change. See the current products' Terms & Conditions on the issuing banks' websites.