What would my best choice be for joining the military?

I am thinking about joining the military but I am not sure what to do. I want to be out on the front lines with the marines. I WAS thinking about joining the Navy and going to be an FMF Corpsman with their Infantry or Recon but I have been looking around and found that it is not too likely that I can be either of those because they are very hard to get into. So, I was also thinking that I should just join the marines. What do you think I should do?