You need to determine a goal and pick a day 6 months or so from now and get to work. You're pretty thin and I imagine you wanting to build up those beach muscles. That's fine but remember overall is better. I have a buddy who never, NEVER works his legs out and looks weird cause he's jacked up top and it's like he's on toothpicks.
First: What does your diet look like. 5-6 small meals per day but make sure you are taking in a lot of protein. You'll need those carbs too. I suspect you have a really low body fat percentage to be your height and be so light (no offense bro). This is going to be thanks to your metabolism being sky high. That's good and all, but everything you eat your body is burning off before it can be used to build muscle. Since you're not really getting very many extra grams of protein from your metabolism you'll need to get some oils in your diet. Start eating salads with every meal with an OIL for dressing. Not ranch. You need to consume about 1-1.5 grams of protein per pound of lean muscle each day. Also, you need to think about pre-, intra-, and post-workout supplements. I personally like Beta-Nox for my pre but don't expect anything as far as taste goes. It's pretty bad tasting but I can feel it working in about 10 min. And no I don't work for them. I'm a firefighter.
Start going to the gym with a friend. Set the same goal and hold eachother accountable. It makes getting to the gym a lot easier. don't worry about any physical differences. it's not about that. it's about saying "dude where were you??" and trust me you'll never want to hear it.
Keep a journal of your weight, the amout you're lifting, reps, sets everything. It's a good motivator when you look back 2 or 3 months from now and see the improvments.
You need to get into a routine but you will plataue if you never change up your exercises.
Bench: 3 sets 8, 10, or 12 reps per set on a decent weight. try to burn out on your last few reps on your last set. it'll take a few times to get it right. over time it will take more and more weight to acheive this.
Same thing goes for the following:
lats: (seated pull down) 3X8-12
shoulders, quades, hams, tris, bis, calves all 3x8-12
Do pushups in the morning as you roll out of bed until you burn out. Just start and don't stop until you can't do any more.
STRETCH!!!!!! nothing is worse than a heavy lifter who cant touch his toes. Losen those hamstrings and you will significantly decrease the risk of slipping a disk later in life. Stretch cold and while they are hot (postworkout)
Hope this helps.