Hey bud, I am a personal trainer and my advice is simple. If you don't know what you are doing work with the trainer 2-3 times per week so they can coach you on all the exercises that you should be doing. Have them explain to you what each part of the workout is for, such as why you are performing 12 reps as opposed to 6 reps, what type of rest periods do they recommend between sets and why, and most importantly what each of the exercises you are performing does for you. Each muscle group can be hit by a vast array of different exercises and many of them will focus more on one aspect of the muscle group such as Decline Bench Press which works your lower pecs more effectively as opposed to incline bench press which will work your upper pecs more. When you feel comfortable walking into the gym knowing what you are going to do and how it is going to effect you that is when you should stop working with the trainer.
If your trainer is always around at the time of day that you go in, and is open to answering questions you may be able to hire him to train you once per week making sure that he instructs you on the exercises that you'll be doing until you meet again. I say this because if he is there on days you don't work together and you have a question, you would be able to go ask him what to do and he can help you, most trainers don't mind giving a little bit of free advice to a paying customer even when you aren't training.
I have left a number of things out but like I said I wanted to keep it as simple as possible. If you have questions let me know.
