How can I improve in my overhand serve?

I'm going into high school as a freshmen. And I decided to play volleyball. Even though I only have a little experience. I do okay in setting and bumping. But when it comes to overhand serving it seems like the ball wont go over the net. Also I feel like I have no strength when I hit the ball. I watched youtube videos on how to do it. I have the format, but I can't seem to do it. By the end of this summer I hope to at least make the freshmen team when tryouts come. Any advice? I know you have to practice alot but any advice. I've read alot of articles but it doesnt seem to help me!