I have had a credit card for about 3 weeks.

Other than that, I have no history of borrowing or making monthly payments. (I bought all my cars in cash, I never rented, and never even had a cell phone bill.)

I talked to my credit union that I have been using since I was 18 (I'm 21 now) and I tried to get pre-approved for a mortgage. The lady told me that it is not possible for me to get a home loan because of my lack of credit history. She told me to use my credit card and try again in 6 months.. I read online it can take 12 months to build enough credit history.

Am I stuck for now? I was hoping I could get something started earlier.

Do you have a stable job history?  Do you have a job?  :)  Will your income support a mortgage payment?

Go to LendingTree.com or QuickenLoans.com or both

Enter your information. It will provide you with a list of lenders who you qualify for. You should qualify for an FHA insured first time homebuyer mortgage. However, be aware there will be a residency requirement.

Buy everything on credit card. Don't worry, I hate owing money (other than on houses) but I deal with my angst by paying it off weekly. Showing you use credit, even small amounts, and that you pay it off in full regularly will go a long way.

If you're living at home, what about putting one of the utility accounts in your name.

@Kyle R. I have opened up 3 credit cards in the last year and half just to build my credit. I was in a similar situation you were in because I purchased schooling and car with cash and lived at home or with relatives till I moved out and purchased my current residence. I had very little credit at the time when I purchased. Literally 2 cc's. Just for safety net reasons alone I have since establish more credit. As long as you are responsible more credit should help you. If it take 6months-1year no worries you are way younger then 98% of the people investing. My advice, don't feel rushed you have plenty of time. With that being said keep moving forward, REI is a process. Any obstacle becomes the path for those that succeed. Best of luck! Stay active on BP, it will produce amazing returns.