Enrollment is different for every place. You typically have 30 days to enroll for your benefits from your start date.

For your medical plan, it's hard to tell you which one to pick based on the info given. When you sign up for it, it will give you the cost (premium) of each one. If you want the one that's cheapest, you can select it from there. Picking the cheapest option will mean that you'll have a high deductible though. Your deductible is what you'll pay in full before your plan starts to help out. Once you hit your deductible, if you're on a cheaper plan, your plan will generally pay 70% - 80% of your medical costs.

If you are a generally healthy person then it can work out for you to go with a cheaper plan as you'll still get discounts for routine / preventative care. But if you get sick a lot or are anticipating surgery you'll probably save money with the more expensive plan.

If you have any questions or want me to elaborate more on anything let me know