What classes/courses you'd need would depend on what unit you would want to go to.

For example, if you wanted to get into Behavioural Analysis, you'd need psychology for sure and idk what else (haven't looked into it). And keep in mind it's nowhere near as badass and glamorous as it is on tv.

Learn a foreign language
Don't have ANY felonies
Don't have ANY crimes period
Have a solid educational record

Those will all be helpful, but the no criminal record thing is kinda mandatory. Also, joining the military will also help.