You know, I'm not nearly as big as I'd like to be, and I'm working on it, but god knows I'm in the best shape I've ever been in now.
I was skin and bones for most of my life, even though I thought I ate heaps and went to the gym regularly.
What I think finally started putting some muscle on was a 30-minute full-body workout recommended by Men's Health Magazine here in Australia. It was only 3 days a week, maybe 4, but it came with a whole lot of seemingly good reasoning.
By working out your whole body with large muscle group exercises, you would stimulate the largest release of human growth hormone which is what you need in order to get bigger. But you also need enough protein in your diet - and it's not really heaps, something like 1.5g for every kg of your body weight, reasonable quantities of good carbohydrates, and plenty of sleep. The rest days after each workout were part of the secret because the growth occurs during rest periods.
Now, I'd prefer to get this sort of information from a muscle-hunk who'd built up from skin and bones, and I suspect you would too, but take my advice as you will, but it has worked well for me - if only you knew how skinny I was to start with. Now i'm on the RJ muscle building program and I like it.