1. You need to track the hours and submit them to the payroll service. I know with GTM Payroll you can set standard hours for the pay period and it'll automatically process each pay period based on your standard hours. But if the hours differ, you'll need to submit.
2. The payroll service should calculate for overtime but confirm that.
3. Depending on what level of service you select, taxes should be remitted on your behalf on a quarterly basis.
4. Look into their year-end service. They should provide W-2, W-3 and Schedule H. There may be an additional charge but could be worth it.