You must be E-5 (or E-4 who's for advancement) or above to apply for that rating. You submit a package through you chain of command and wait for it it be approved according the needs of the navy.
In the mean time, try to enter the navy with a technical rating (AT, ET, CTT, CTM, CTN, CTR, FC, IT) since they tend to be undermanned in these recent times, and after getting E-5, apply to be a LN. The more undermanned is the rating, the higher your chances in getting E-5 quicker. However, some of these ratings require a six year enlistment, and since you also had some credits you may be able to go to bootcamp as an E-2 (24-47 credits) or E-3 (48 or more credits).
More info in these links;
http://www.jag.navy.mil/documents/in...01440%201D.pdf
Converting to the Legalman Rate
Have a lovely day and I hope to see you soon here in the navy.