If you're looking for a playbook, it depends on what kind of offense you want to run. But you're not going to find a quality one for free.
I don't mean to be short with you or sound like a dick, but football's a very complex and deep sport and no one book is going to cover everything you need. If you're looking strictly for offensive stuff, if you can, I highly suggest trying to find out what offenses are popular in your conference. And then running something totally different. I'm a big believer in going against the norm in football. It confuses kids and causes coaches to spend extra time scouting and planning specifically for your team.
If you can't find out, just implement something uncommon. Maybe single wing, double wing (this may or may not be popular in your area), lonesome polecat (probably best to stay away from this one, fun to play with, but easy to screw up), or whatever. Personally, I'm a big fan of the single wing, but it's getting harder to find resources for it. But, like the guy above said, your offense has to match your athletes.
You might have to settle on something a little more standard, like a wing-t or triple option (requires a lot of practice with QB reads and moves to really run effectively). Whatever you go with, I suggest focusing on the run unless you have a stud QB. Too many bad things can happen when kids try to throw. It's best to limit those things.
Once you settle on your offense, study the hell out of it, for yours and your players sake. You can't be mad at them for not knowing what to do in a certain situation, when you don't even know what to do in that situation. So learn it and master and, if you can, read about it from several different sources.
After that, decide on how large you want your playbook to be, then cut it in half. Seriously. You're not playing with or against pros. You don't need a large and overly complex playbook. Pick 6-12 plays and coach the hell out of them. It might not seem like enough, but if done properly, it is. For example, if you have 6 plays, you can run them either left or right. Then you can run them out of 2-4 different formations. Depending on your offensive pace, that might be enough for an entire game without repeating an exact play. But, your kids have to know these 6 plays inside and out. You can also ass stuff like motion and shifts to further disguise your plays.
I once coached a team of freshman with only 5 plays. Of course will need to have a few situational plays like hail marys, taking a knee, etc. (Yes, you need to coach taking a knee. If you don't someone will screw it up.)
Anyway, this is a start. Coaching is fun and rewarding, but it's not easy. Coaching is far too complex to waltz into with only good intentions and some playing experience. Good luck.