For both cardio and weight training, it gets exponentially easier and exponentially fun. What this means is it's a total bitch to start. I started with bike riding, since it's probably the easiest to start with and won't make you feel like total crap when you start. It also gives the benefit of solidifying your thighs, if you want that. I had a Cyprus Giant bike, which leaned more toward being a tour bike than a dirt bike. Either works fine, but being able to switch gears gives the added option of changing resistance.
If you are a man (or woman) of strong willpower, do lap swimming. I say strong willpower, because it's going to suck ass for the first two weeks. I believe I did freestyle swimming, the kind you see most people do. It seems to focus on your shoulders, arms, and tightens your abs. The breast stroke will strongly tone your chest. When I was 16 and 180 pounds, I couldn't go more than one lap and back the first time. Yes, I couldn't swim more than 50 feet without getting winded. By the second week, I could go constantly for exponentially longer amounts of time. At my peak, I could swim a full two and a half workout without being miserable. (for the record, I atrophied almost back to square one, hence OP Cardio)
Running is the same. At first I became easily tired, but by the end of the season, I could jog and run for hours on end, with reasonable walking time every once in a while.
Cardio stacks up. Swimming will improve your running, and your biking. It looks like for a 17 year old male, it takes about two weeks to break in from a sedentary lifestyle to an active one, and for one muscle group to get used to a certain activity. If you ran for a full season and decided to swim for the next, you'll still suck the first few times, but the previous season's work will up the exponent factor.
When I was 16, I exercised sporatically. My doctor, magazines, personal trainers, and such are mixed on how often to do it. I found success later on in exercising every other day, Although I don't think it really matters. Compare swimming every other day to Basic Training. Unless you are swimming to the brink of needing the lifeguard to valiantly rescue you to end your training session, you probably don't have to have a mandatory day off. Think of the Army dudes and chicks in training. They are working their ass off to the Bone every day with four hours of sleep! Obviously, they're doing something right. It may depend on how intense your cardio workout is, and chances are you will be underworking yourself than overworking.
Lifting is the most grind-like. Ask yourself, 'what part of me do I want to turn into sexy, rippling perfection?' and go from there. At 16, I had a 15 pound freeweight to start out with. At the very beginning, I curled that sucker until I just couldn't lift it any more. Like cardio, it's an exponential thing. I started with lifting a 15 pound weight with one arm a petty two times! By three seasons, I could do a rep of the 15 pound weight 35 times! Since I was poor at the time, I did floor exercises and isometrics. Floor exercises are your basic push-ups and sit-ups, along with leg lifts. Regular push-ups focus your chest and arms, knuckle push-ups focuses entirely on your arms, and wide-space push-ups focus your chest. Lol my first knuckle push-up was a half of one, and worked my way up to nearly thirty before I stopped. Sit-ups and leg lifts are very important for abs. My younger focus were primarily midsection and biceps/triceps. Normal sit-ups focus your upper 4 pack, and leg lifts focus your lower 2(4?) pack and direct upper groinal area. Professionals are mixed on sit-ups. If you do them right, you will get a masochistically pleasurable 'paingasm' in your abs, which oddly feels better than it sounds, and if you do them wrong, your lower back will just hurt. Ironically, if you spread it between both your abs and your back, you'll get some definition back there. As a man of particular taste, I wanted the full midsection experience, so I also worked on obliques, they are the muscles on the side of your abs, if I'm not mistaken, where your kidneys are. You can include them into the mix by changing your sit-ups to placing your hands behind your head, elbows forward, and touching your elbows to a knee diagonally. Alternate between sides.
I now have a gym pass, so I use machines now, although I will probably still do sit-ups freely. Push-ups ended up giving me a really bad migraine, called lifter's headache, and can sometimes be associated with doing push-ups or bench presses wrong. I may try push-ups again, but the machine got me covered. The machines are self-explanatory. For muscle mass, do two sets of weight pushing your limit, or one super set of really pushing your limit. For tone, do three or four sets of whatever you can do for that much. Remember, tone looks just as good as musclely (most women agree too
) Your cardio alone will help tone you too.
A word on bench presses. Bench pressing is the classic thing where you lay on a bench with your feet on the ground, and you raise and lower a bar with comically large (or even more comically small) weights on the end. It improves your chest, and can be changed the same way as push-ups. Normal hand spacing is having the ends of your hands flush with little etched (or non-etched) rings on both sides. Bringing them closer focuses on arms, and really spacing them out focuses the chest, or armpits. I would strongly suggest having a spotter for doing bench presses. Benching works best if you do it until you simply cannot lift the sucker anymore, which is obviously very f*n dangerous if you give it the extra push and cannot bring the bar back to the starting clips. It will have nowhere to go but down. Down to your chest, down to your neck, with you having no more strengh to do anything about it. People die from this on a regular basis. Most high school gyms and some other gyms metal rods that run parallel on both sides of the bench that you can adjust as a minimum lowering point. Obviously, it's better to have a giant heavy rod resting an emergency rod than on you. The drawback of emergency bars is people often take the weight bar all the way to their chest to get the full range of muscle movement, so ONLY remove the emergency bars if you have a spotter. and obviously, if your spotter is much weaker than you, make sure either he can squat that much weight back up, or both of you can lift it between you two. Hilarious failures of this can be seen on youtube to demonstrate the point.
Pull-ups are great for biceps and triceps. Palms toward you build meaty lumps on the inside of your arm (biceps), where palms away from you help remove the disgusting flab that hangs by your elbow, replacing it with sexy, meaty flesh (triceps). Be warned! Doing many of each will give you an unsettling ability to climb things, even with your arms alone. When I was younger and retarded, I found I could climb trees very easily and more stupidly, I could climb street lights and flagpoles, much to the amusement of others.