Vacine,
Congrats on your solo! Sounds like you did very well. I fly for a living in addition to R/C, and the R/C aircraft behave just like the big ones. An airplane is an airplane no matter the size. I would have to say that your left turn was caused exactly for the reasons tinman suggested. Wind will get underneath the upwind wing if you don't give the airplane a little bit of aileron input into the wind. If you imagine yourself in the cockpit, just put a little bit of pressure on the stick in the direction the wind is coming FROM. Just how much is necessary is a function of how strong the wind is. As a beginner, you will not be flying on very windy days, so a little bit will be more than enough. As the airplane picks up speed, less aileron is required. You have to "feel" how much you need. Airplanes with props that turn clockwise from the cockpit perspective like to turn to the left due to torque and something called "P-Factor" This causes the airplane to prefer to turn left even in calm wind conditions. This is more pronounced at slow airspeeds and high power settings...exactly those conditions found during takeoff. A little right rudder is normally used during takeoff. Not much, but enough to correct the problem. It's a lot easier said than done especially in R/C airplanes. Pick up a book on basic aerodynamics and you can learn a lot that will help you out, and it will be explained for you much better than we can do here. Again, congratulations on your solo! Enjoy!