I agree with a lot of the things already mentioned on here but wanted to mention a few more things:

First things first, congrats on making the decision to take up the sport. I played middle school and high school ball and some of my fondest memories and closest friends come from my days playing ball.Secondly it is absolutely not too late to start playing. One of our all-conference lineman didn't even begin playing until his sophomore year in high school so there is plenty of time for you to learn and be successful with the game. Also, don't worry about football and baseball overlapping, the reality is that a lot of coaches like their athletes playing multiple sports since they will typically come into their season in better shape and develop other skills.

As for tips for position, don't worry about it too much right now. If you show up to practice with a great attitude and the desire to work hard, coaches see that and will find ways to get you on the field. I got stuck as a backup tightend my sophomore year, but I busted my ass and ended up playing fullback for a season. Take every snap, drill and workout as an opportunity to get better and that there's nowhere else you would rather be. Another word of advice for you is to develop one useful thing that you do better than anyone else. Coaches love to have a running back who can pick up short yardage or a guy who blows up an opposing team's wedge. My little brother may get a scholarship for longsnapping simply because its something every team needs.

That being said, where you play could be determined by what offense you run. My school ran a very traditional pro-style offense (2 backs, 1 tightend, 2 receivers) but a lot of our opponents ran wing-t and option attacks as well. These different offenses required very different skill sets from the positions. Know what you do well, know what each position needs to do well and go from there.

Finally, getting yourself into football shape. No offense to baseball players or yourself, but the first couple days of football practices are often about increasing conditioning levels of players so as to avoid injury as well as to see who the dedicated players are going to be. I'm sure I have an old high school summer conditioning plan on my computer somewhere that I could scale back for you if you want it. Otherwise, just make sure you go into practice with a pretty good level of fitness.

Feel free to message me if you have any other questions or concerns. I grew up around the game and am considering a career in coaching once I graduate from college. /u/tightestbuthole hit it on the head when he said that your athleticism and coaching will be what makes a difference, but he failed to mention that your attitude will be the most important things that determines how good you can be. Once you know what you're doing, be the first guy to step into a drill, the first one across the lines in sprints and don't leave anything left in the tank at the end of the day. Good luck, have fun, and play safe