All potential candidates must take Physical Screening Test (PST). Even if you're not training to be a SEAL, the PST makes for a good benchmark of your own fitness, and can be used as a quick, full body workout that requires little equipment. Here are the minimum requirements from the official Navy SEALs website:
500-yard swim using breast and/or sidestroke in less than 12 minutes and 30 seconds
10-minute rest
Perform a minimum of 42 push-ups in 2 minutes
2-minute rest
Perform a minimum of 50 sit-ups in 2 minutes
2-minute rest
Perform a minimum of 6 pull-ups (no time limit)
10-minute rest
Run 1 ½ miles wearing RUNNING SHOES and SHORTS in under 11 minutes
Remember, these are the minimum requirements for the PST. Challenge yourself to go above and beyond and you'll see more significant improvements.