Infantry is a good choice.
You will hate much of the job, hate the waiting, hate the people in charge of you, but will make great friends.
Work on your running. The rest of it will improve eventually. If you can run you'll get left alone for the most part unless you have a big mouth.
Hit up military.com and read some of their stuff, it will help you.
Familiarize yourself with army ranks, you'll need to know them. Work on your running. If you are a fast runner, practice slow runs. 5 miles at 8-9 minute mile pace.
If you are fast runner make sure you can do 2 miles in <13:00