It isn't possible to get it written into your contract to be a particular flavor of Navy Nuke. About week 4 of boot camp, all prospective nukes meet with a Nuclear Field Advisor. During this interview, the NFA will discuss with you how you scored on the ASVAB, your transcripts, employment and hobbies, as well as find out what it is you'd like to do. Based on this interview, you will be assigned to one of the 3 nuclear rates; however, keep in mind there is significant cross-training that happens to ensure everyone understands everyone elses job(s).
Also keep in mind the ratio of MM's to EM's to ET's is about 4:3:2, not counting ELTs. Read the other posts on this particular Forum to learn more about what each of the ratings actually does in the power plant, and/or ask your local Navy Nuke. Not sure who that is? Ask your recruiter, or e-mail me with your location and I'll send you the contact information.
As a submarine nuke ET, I can describe what I've done over the past 20 years (both at sea and on shore), but I'm sure my biases would shine through in my descriptions of what the other submarine ratings do...
Nuke1_port@cnrc.navy.mil