Yes, you will be automatically enrolled in Parts A & B. You can opt-out of part B but in your case not advisable unless you can and want to continue your company benefits. You're medicare won't take effect for 5 or 6 months, just about the time your company benefits end. You'll be getting a letter from SS giving you an exact date.
If you decide to stay on straight Medicare you will need a part D coverage. If you want to go with a Medicare Advantage plan which is an HMO provided by many companies including Aetna, Blue Shield, etc, your prescriptions are usually covered by the HMO, so you don't need to pay for Part D. Most of the HMOs require no additional premium beyond the straight Medicare premium.
Some people don't like the restrictiveness of an HMO and I can understand that. But it works for me and is less costly than straight Medicare because I don't pay 20% of my doctor visits.
Another option is Medicare with a supplemental. But if you're under age 65 this is going to be expensive. For me, at age 55 it was approx $330/mo extra on top of Medicare premium for a Plan F type of coverage (full coverage). At that rate, it's better to keep your company plan and go on Cobra.
Hope this helps.
-Tom