IF you have bad credit, you can't improve it by paying cash. You don't have credit. Get a secured loan. Basically, you give the bank the money to sit on and you make payments. In a year, you pay off the car and show a nice positive mark on your credit score.
If you have a credit card, use it for ALL of your bills, but pay off every month. If you're serious about buying a house in a year, what you boost your credit by will make up the difference in the interest rate they charge you.
And, pay ALL YOUR BILL IN FULL EVERY MONTH. Every time you pay late, remember that shyt stays on your credit score for 7 years.