I didn’t read any replies so if this is a repeat, sorry 
First of all, Wu Wei, go search through the old threads here. I’m pretty sure you can come up with a good routine if you do that (especially if you read my old posts, haha). No I’m serious. I wrote out a whole friggin’ routine for someone once, and even gave page numbers where the exercises could be found in the “Big Arnold Book.”
But listen dude, you sound naturally skinny like me. Welcome to the world of ectomorphs. We have some advantages, like we can eat whatever we want and not get fat. Also, in school we never got picked on for being the “fat kids.” However, there are some disadvantages, too. Like, um, it’s bloody hard to build substantial muscle mass 
Here’s how your body works. If you burn more calories than you consume, you will lose weight. If you consume more than you burn, you will gain weight. Simple, right? Being an ectomorph, your metabolism will go crazy and burn whatever you eat, so, the first rule for you to gain weight is:
EAT A LOT
What you think is a lot is not a lot. A lot of food is 5 or 6 1,000 calorie meals a day. A lot of food is having to take 2 or 3 crà ps a day. A lot of food is not being hungry and eating anyway.
Hehe, but listen, dude. You don’t want to put on too much mass, because it becomes unhealthy after a point. For example, look at pro bodybuilders. They LOOK big, but they are really, really unhealthy people for the most part. Many have colon problems from constantly eating thousands of calories a day for years. They have miles of extra blood vessels from drug use over the years. They can barely walk across the stage without getting severly winded.
Ok, you don’t want to get that big. Good. A BB who can’t even wipe his own arse wouldn’t make much of a fighter.
But, the principles for gaining muscle mass remain the same. You must have a caloric surpluss in your body.
You must also work out with more sets than if you were training purely for strength. You will be sore for 2 or 3 days after a workout. Make sure, as stated above, you eat about 1 gram of protein per pound of bodyweight. The workouts will, in essence, direct the protein to your muscles instead of being excreted or stored.
Listen, don’t start eating like a madman because you will get sick. I have no idea how much you eat now, but try just adding another meal each day for starters.
It’s very possible to add a healthy amount of muscle, the only drawback is you MUST eat more to add/maintain it. Being an ectomorph, this will be a bit difficult compared with other people. However, if you are a true ectomorph, your bodyfat is already low, and whatever muscle you add will look ripped.
You’ll be a stud.
Haha.
Good luck, dude.
Iron