Good Job
First Game... Good Job. I love tower defence games. Since its your first game I am not rating to low but i have a couple points of advise that should make your games even better.
1.) Add a menu screen. And have a Level Select. So that way your players are noit stuck playing on only that level. They get a choice.
2.) Have a menu button to go back to the menu just in case they want to go back.
3.) If you have the money hire an artist and get the game sponsered if possible. I know its hard to draw on the computer but the better the graphics the better the gameplay. If it is sponsered it will make you money to.
4.) Personally, I like the tower defence games where you dont get to chose when the wave comes. It should automatically come after a certain period of time along with the option to make it come right away if you want.
5.) Keep making games. Especially if you get them sponsered. Good way to make money. And release your creative side
Good Luck