How can I obtain a Top Security Clearance?

If the MOS you request requires one, and you meet all the other prerequisites for the MOS, your career counselor will provide you a memo to give to your S2. This memo is the justification required to initiate a clearance.
You only need an interim to attend training.
I assume you haven't talked to your career counselor about Reclass yet because they should have given you the memo.

There is a computer system I believe it is called EQuip. In any case, your command has to support eligeability as it costs a great deal to complete. The reason they need to be supportive is they will be listed on the computerized document. If you are otherwise eligeable for the new MOS, they will likely support the transition. Then it is a matter of 6 months or so, + or - to complete, which can also impact your eligeability for the transition. Good luck and thank you for your service.

See your retention NCO. He or she will have you fill out an SF 86. Be truthful on everything. It will be double and triple checked and investigated thoroughly.

After that you have to wait. Depending on how far back and how much they have to do will determine the length of time it takes.