Ok I am also about 1.7m, and I used to be in the region of 70kgs. I managed to up my body weight to around 84kgs now, which is in the comfortable zone for my body. Anything more is actually getting fat. The main thing is you want to LOOK HEALTHY - no matter what your actual weight, it's how you LOOK that counts (unless your goal is to train, but it seems like you're more looks conscious than sporty). Don't pay that much attention to the scales, but do pay attention to what people say about you. If they start to comment on your spare tyre, then you know that you need to do something to lose the weight. I had some comments over Christmas time, and I've lost three or so k's since then, and people have commented about it.
So to bulk up hit the free weights. Here's my usual gym routine - I do about 15 minutes of running, 5-10 mins of abs then maybe half an hour of weights in whatever order I am working on three times a week. I've found that's good enough for me - anyway it's what I can fit into my schedule.
The best 'bulk' exercise for me is old fashioned dumbbell bench presses. Start wherever you are - probably around 10-15kgs I'd guess - and do three reps of 10, building weight to your limit every time. Also dumbbell curls and bar curls are good. Make sure not to swing your hips when you do bar curls, just focus on crunching your biceps.
And please don't neglect your legs or your back. Seated leg presses are great. Squat thrusts will give you a solid back.
Diet wise it depends how intense you want to get. You can just eat everything but in smaller portions, which is what I've had to do recently. You'll be really surprised how quickly the fat sheds off by just eating a bit less. Forget about steroids, protein shakes are supposed to be good but I'm really not that hardcore to try and find out.
Whoever said that once you hit 23 you've lost the opportunity to build up your body is talking rubbish. I didn't really exercise till I was around 27 or so. It is harder, but it can be done. Not that I'm anything great really, but I have come quite a long way since I started. You couldn't imagine how out of shape I used to be. Good luck!