Some really good advice here mate... But take that BMI chart and throw it in the trash where it belongs! I'm 6' 220lbs which is WAY aver where I should be according to that chart. It says i should be 190lbs... no thanks.
Everyone is different. It's great to use charts, graphs, and workout plans as a guide, but you really have to do what's best for your body. Sadly, it's all about trial and error. You're going to have to put some work into it and help your body figure out what works best for YOU.
At 6' 220lbs, I can run 5K in under 30 mins (which isn't bad) do pushups and situps til the cows come home. I'm not saying everyone who is 6' should bulk up, but it just goes to show how everyone is different compared to what some chart says.
6'3" 160lbs, your metabolism is SICK! As a personal trainer and someone with certs in nutrition, the first thing I'd say is EAT.
Heard it before... I know. But what I mean, is you really have to load up the calories.
It was funny when I went through the interview process and the gentleman gave me the list of drugs and all the other illegal items on it. I filled out that I have taken steroids and other prohormones when I was a kid. It didn't seem to be a big deal since it's been about 12 years since I tried any of that stuff.
I'm not saying you should try any of that garbage (and that's what it is, garbage) but doing that stuff showed me exactly how my body worked. To build muscle, I needed food... and lots of it. There were times when I ate well over 4000 calories in a day. It seemed like all I did was eat, sleep, workout, and go to school. sadly, it was exactly like that. Even at 245lbs, I had trouble staying big if I didn't eat enough.
SO, of course, once I smartened up and got away from those chemicals, I continued to work out and eat right. I trimmed out and kept some really thick/dense muscle. And although many people who have just as many certifications as I do may completely oppose what I have to say, All I can say is that it worked for me and I'm sure if you get to know your body a little, it may work for you...
I believe in a slow gain. Don't rush out and start eating 5000 calories a day. Take a week, write down everytihng you eat and then put it into a calorie program like fitday.com (I love this one) Next, look at your scores. How many calories are you eating per day? what is the ratio between protein/carbs/fat? Then, decide where changes must be made.
(again, I'm not telling you that you should o this, but this is what worked for me)
A typical protein/carb/fat ratio should be 40%/40%/20%
= 100% just so we're on the same page
There will be a small pie chart to show you the %'s
Next, let's say you're only eating 1500-1900 calories per day. Well, for someone who wants to gain weight (mostly muscle mass cuz let's be honest, you're going to gain fat too) you're going to have to eat more!!
Try adding 150 calories into your diet each week until you start to see some gains. I know, it's slow, but it's better than adding 500 a week and ending up with sick amounts of bodyfat and liver issues...
Keep doing that until you start to see real gains in your strength and muscle mass. Use the mirror, scales are a pain and they just get in the way of real training. But,if you need that sort of stimulus... weigh yourself once a month.
As for the workout routine... it's hit or miss. Low rep/heavy weight or moderate rep/moderate weight... Pick a weight and do 10 reps. If you struggle to do 10, then that's your weight and your goal is 12!
When 12 gets easy... increase the weight. 3-4 sets are fine... don't spend your life in the gym. Try to be in and out of there in an hour unless you do cardio...
That's basically it. Oh, and take a multi vitamin! lol
the rest is up to you and your body.. it'll let you now what's working.
Good luck!