Can someone help me on how the draft works. Do you have to be on during your pick?

Your pick will be selected at the time indicated in the schedule (on the war room page). The schedule is set at the end of early free agency.

You don't have to be online to make your pick, but you should try to set up your draft board to your liking. You can adjust the auto-queue list with the 'edit' button above your draft board and set weights and goal counts for each position. I usually leave goal counts at 0 but you can experiment with how it tweaks your draft board (when you save it your draft board will update). You can also drag players into the manual list which will put them at the top of the draft board. If you'll be online near your pick, you will probably want to use the manual draft board, unless you can play with the auto draft to get to your liking.

When your time limit hits, whoever is on the top of your draft board will be selected and automatically signed to your team. If for some reason you draft a player that you do not want, you have until Training Camp to void his contract which will allow you to release him without taking the salary cap hit from his bonus. If you do this, however, you are forfeiting that pick.