I'm sorry, but this game isn't very good. There are many issues, such as there being no way to get back to the title screen from the menu, and there's no explanation of the controls. But there's 2 major issues that I noticed. The first thing is the controls. You use mouse to move, but spacebar to shoot. Why don't you just use right click to shoot? it's really hard to shoot with spacebar, aim using the mouse, and move at the same time. Also, the art looks like it was done in Microsoft Paint. It just looks bad.
However, I recognize that this is probably your first game, and for a first game, it's not bad. You need to spend more time fixing bugs and doing the art for your games, though.