Either have your UTM set you up with Base Training or call them yourself. First and foremost, you need to be eligible to re-train. Since you're a SrA, I assume you're not in upgrade training for your 7-level yet and you already have your 5-level in your current AFSC, so that's good. Typically you can't leave an undermanned career field, but TACP and other battlefield airman AFSCs have such a high washout rate, they mostly expect you to end up back in your PAFSC anyway, so that's less of an issue. Still, if you're currently in a critical career field, you may not be allowed to re-train.
You need enough retention left on your contract to complete tech school and still have two years remaining on your enlistment. You will almost certainly have to re-up or extend your current enlistment. You can't have any pending disciplinary action or an open UIF. You'll need your supervisor's recommendation and commander's approval. You have to pass the same medical and physical standards as for PJs, CCT, and combat weather. Unless something has changed, you have to clear the minimum for TACP on the PAST test as well. Base Training can fill you in more on the specific steps.
Former USAF flight medic and Unit Training Manager