If I were going to invest 10k, no matter if I borrowed on a credit card or had it in the bank, I would probably just find one flip deal, find a money partner (maybe on this site?), use the 10k as "skin in the game" with the partner, and do the deal with YOU as the buying agent when you get in, manage the renovations/contractors to earn your slice of the profit, and be the listing agent when you get out (maybe even at 2% to make your partner happy). You will probably get most or all of your 10k back on that one deal just in commissions and maybe a finders fee if you're lucky. If you did it right, you should also have at least a few thousand in profit from the flip. All in about 2-3 months. Now you have built some connections, you have investing experience to talk about, you really understand the costs and effort involved, you have some cash to play with, and presumably during that time you were lining up your next deal (or 2 or 3) and are ready to do it again, and again, and "grow" into the business you are interested in doing, while also growing your bankroll and your network.
At 19, your mind is probably going faster than reality will allow you to go. I would never want to hold back enthusiasm, but 2-3 months of patient grinding is probably what you really need right now (what I would give to be 19 again). The last thing you need to do is invest (or borrow) 10k into mail that is probably going to be very frustrating and slow, if it generates any leads at all. Most direct mail takes 3 or 4 rounds before you start to see results, so that could be months. Go out and just start talking to people. Join a local Toastmasters and talk about what you are doing. If you go to church, there is another source of leads. Check out local meetups and just talk, talk, talk about what you are looking for. It won't take long to find a deal worth doing. When someone says "Hey, I have a house I need to sell..." then you listen, and don't let them out of your sight until you either have a deal or know there is not a deal to be had.
By the way, I am a Realtor in Arlington, TX, but I hope you won't hold that against me. I am also an investor. One good contact is all it takes to get the ball rolling. Some nice lady whose husband just passed away and needs to sell the house as-is quick. An angry husband going through a divorce that is ready to sell the marital home right NOW so he can move back to Nebraska and grow lima beans. A probate lawyer that has a client/executor that needs to liquidate the house of a hoarder for out-of-town estranged kids. A part-time out of town landlord that has had it up to here with those dang renters. You never know where a deal is lurking. Talk. And listen. And act.
Anyway, those are my thoughts. Maybe I got carried away.