That's a difficult question on several fronts. Some of the initial hurtles are 1. Do YOU qualify mentally, morally and physically to enlist in the National Guard? Let's for the purposes of this forum assume that you do. 2. Depending where you are located, are there any 68W vacancies in your state? (ie are there any job openings?) That's a big question these days because frankly alot of units are pretty..well, full. Finally, to address the part of the question you might really be asking...or even if you're not...if you are a currently registered EMT-B, it can allow you to "skip" the first 5 weeks of 68W AIT. (That's when your would be classmates are taking the EMT portion of the course).
First you would need to qualify for 68W on the ASVAB. Then to fast track through 68W's EMT certification portion it would need to be a National certification and be current. State certifications can try to challenge it.