For conventional purposes you are technically not a first-time homebuyer if you have had an ownership interest in a property at any point during the last three years. However, I am curious about why you're asking and assume it's because the loan for which you co-signed may now be impeding your ability to get a loan on your own. If this is the case, you must follow Annette's advice below, or have the primary borrowers refinance that loan in their names alone. That would be the way you'd get out of the debt obligation.
Whether or not you're a first-time buyer should have no impact on the requirement to qualify with an acceptable debt-to-income (DTI) ratio.
If you deducted any mortgage interest or real estate tax in the last 3 years, you are not considered a first time buyer.
Or you on the title? If so, you are not a first time buyer.
With that said you will qualify for a FHA mortgage (3.5% down) if you can prove that someone else is paying the mortgage.