If you have a credit card already and want a lower interest rate on that credit card you need to be with the credit card company and have a credit card account good standing with them and pay your credit card bills on time and keep a low balance on the credit card to increase your chances of getting a lower interest rate.
I would recommend waiting until you've been with the credit card company for 2 to 3 years before trying to get a lower interest rate as this allows you time to use the card responsibly and build up a good payment and credit history.
Then call the customer service number on your credit card and speak to a customer service agent and ask them if it's possible to negotiate and get a lower interest rate.
Sometimes you may get the lower interest rate and other times you may not but you can keep trying and be polite to the customer service agent even if they cannot get you a lower interest rate at this time because they're only allowed to do what they can by their boss.