Martial art best suited for me?

Am quite interested in taking up some form of self defence or martial arts... Am quite little guy and looking to get fit and leArn some defense that won't be too stressful but I want a workout, I am don't know much bout them but was looking at bjj or judo, which might help my afl tackling, mainly for fitness any recommendations? Am only 16 and don't want anything super expensive.. I recognize I should have a practice session or sit in on a cl but just want an idea of things, cheers rupert also am quite interested in boxing but not sure if I'm allowed to and what the cut off age is?