In my middle years I've got aching joints because of overworking the body.
Wish it was possible to go back and do it all over again.

I wouldn't worry about "bulk". A lighter frame will put less stress on the joints over time, and if your genetic metabolism says 125, let it be around 125.
(That is pretty thin, though.)

If you want "strength", then definitly excercise and/or join a program. Swimming will give you tone and long lean muscle mass without the extra bulk. Yoga, pilaties, martial arts will do the same.
Whatever you do, avoid heavy linear repetitions(i.e. max bench pressing on a machine) and stay balanced(don't work solely on the upper torso).
Stretch, and warm up first and start slow. Make it a 2 year plan.

Sorry for the late response, I've had exams all week. Thanks for the suggestions guys, looks like I will get a gym membership and I will workout with my friend who is a trainer. I'll look into those books. I really want to bulk up a bit, the only reason I would stay like this is if I was racing, the weight savings would probably be worth it. ;)

Be thankful for your thin build. I was thin in the 20's. After 40 (and marriage), my body started to change 'shape'. I've always been active, but as I enter the 50's, I'm glad I had a small frame. Most people I've known went from average to 'heavier'. I went from thin to average (5'8" and 145lbs). I've been going to the gym regularly for the past 10 years along with bike & running. Now I manage to keep an 'average' frame and am happy.

Live a healthy lifestyle and don't be obsessed with your weight. Be happy with yourself and don't worry about what other people think.

This explains everything.:D

I should have known.

I like this thought ;)

Self appreciation is the first step to real success :D

Yes, on the gym.

Much good info above. Add'l Tips:

I'd start with machines - switch to free wts. when you get some experience.

Free wts., esp. dumbells will work smaller ("control") muscle groups that are very important -- not just "looks"

HGH and androgens enhance muscle bulk - BUT it's more show than go - your strength does not go up as much. I saw a new research report on this just a few weeks ago.

Start with high reps & low wt. on the machines; gradually work to 3-4 reps of higher wts.

No more than every other day for the wt. training. Alternate with cardio workouts.

Tai chi will build muscles too - even in old people like me.

the most important thing is to be patient and dont give up. building muscle is like running a marathon, not a sprint. its a slow process. I was alot like you when i was your age. I was 5'7 and around 120. now I am 40 and 165 but today I can bench just over 300. Start withe the basics, situps lots of push ups and pull ups work gradually into weight training. Do benches and lots of squats. Protien shakes will help some with the weight gain. A guy 17 years old has an insane amount of natural testosterone so you dont need the juice and dont let anyone convince you that you do. you're already on it..thats part of being 17. post from time to time and let us know how you're doing. I used to fight and coached boxing so i have alot of interest in seeing people train and get in shape. For me its become a way of life. good luck and dont ever give up. you CAN do it.

Don't skip the legs! Research the safety of squats. I used to squat 330lbs ( 6' 200lbs at the time) and I think it is frowned upon now. I tore my acl playing soccer when I was 32 - I believe that my leg routine made my knees less pliable. Just be safe!

I've always felt that body builders were trying very, very hard to compensate for some other perceived shortcoming(s) in their life. I have no evidence of this, and it's not something i've put a whole lot of thought into, but it sure seems to be a common trait in body building types.

I find naturally built people to be the most attractive, and almost invariably, the most confident.

Any time i've lifted, i've lifted for tone, not for bulk.

I suggest you concentrate on the core lifts. Bench, squats, deadlifts, pullups, those build muscle as well as core strenght.
At 6 ft and 125 lbs, you can and should eat anything in sight.. lean meat, fruits and veggies..

I check out these sites:

TESTOSTERONE MUSCLE | Unapologetic Muscle-Building Elitists
More of the hard core articles but the forum posters are normal folks.

Men's Health Magazine : Men's Guide to Fitness, Health, Weight Loss, Nutrition, Sex, Style and Guy Wisdom
Good basic knowledge but they do tend to repeat stuff.

Muscle and Fitness home
Similar to MensHealth but more real I think.

ExRx (Exercise Prescription) on the Net
This is a good over all health site worth exploring.

Here's a link to the workout plan I did when I first started lifting (for health) and had very good results (bulked up/lost weight).
Workout Plans : Men's Health

Remember to "build "muscle you need to:
Lift heavy (try to lift as much as you can in the 6-8 rep range, when you can lift it 10-12 add more)
Rest (work out every other day max or you may over train)
Used compound exercises vs isolation (works more muscles at once and is more natural)
Eat healthy, good food (6 smaller meals rather than 3 big ones)
Whey Protein, multi-vitamins, creatine (no steroids, you don't need them)

That should get you started on the right track.