How can I get faster in a 3 mile race?

The last race 3 mile race I ran was two weeks ago and I ran it in about 20:00:00, last week I ran a 3.4 race with my highschools team and I finished in 23:00:00. I got 12th. In order to get on Varsity, I need to get in the top 7 which is about a minute away from the 3.4 mile race I did. I need any hel-p that I can get, I wouls like to get my 3 mile pace at about 18 minutes by district, which is a month away. By the way Im a freshman so I don't really know names of exersizes. If you give me any, please explain them. Thank you very much.