6 months is plenty of time to make some pretty serious changes.
If you have weight to lose, you can realistically achieve 48 lbs or more (based on 2 lbs of weight loss per week, which is what most doctors believe is sustainable loss).
You also could change your muscle composition quite a lot. You don't mention your age, which is a factor. If you're a teenager you could significantly increase your muscle mass by working out because you may still be having growth surges. If you are an adult, it's likely you could only add 5 to 10 lbs of muscle but you can definitely develop much more muscle definition in 6 months particularly if your body fat is 15% or less.
If you are looking for cardio, this is probably the easiest of all to achieve. 6 months is plenty of time to train to run, for example. Let's say you can currently only run a 15 minute mile- you could halve that time in six months on the proper training program.
Well, I'd guess you need to practice your martial arts, build cardio, and also build muscle strength and flexibility. It might be worth talking to some of the more senior folks at your marital arts school about their fitness regime, or even hiring a personal trainer for a couple sessions, to design the best plan for you.
Still, the basic principles are pretty simple. Figure out what you can do now, then increase it. Ride farther or longer. Increase no more than 10% a week, and don't expect 10% every week. If you lift weights, make sure you use very good form. Have someone teach you how.
I agree with everyone here. 6 months is a long time for fitness gains. You can do a lot! I strongly encourage you NOT to rush it though. Just take your time and go in moderation, with later periods of more intense/thorough training. Just to give you some good news, I was coming off a long injury once and had to sit out and not do anything for about 2 months. 4 months out from a key mile race (conference) was the first I began training. I took my time, didn't push it too hard, kept from getting injured, and almost PR'd in the race. I swear most of the times I didn't get impatient were the best times I had. Have fun with your training regimen and enjoy yourself and just watch yourself GRADUALLY get fitter. It's fun!