Hey whats up guys? Just signed up hoping you guys could help me out.

I'm 19 years old, 5'8 120 lbs, vegetarian (no fish either, but all the dairy is fine). I recently signed up for a membership at my local gym, but have several questions.

I used to work out half ass every now and then, and wouldn't really see gains (obviously), but this summer I'm going to be serious about it and stay consistent. That being said, how can I gain mass and muscle? Can anyone offer a routine for me? Even though I used to workout a bit, and know a few things, I want to do it right this summer. Ideally I would like to weight about 135-140 lbs after 4 months, and have most of that be muscle. Is it possible and how would I go about doing it? I'm on summer vacation right now so I have all the time in the world, and I will be truly dedicated. I know my diet will be very important, but being a vegetarian makes this aspect a little more difficult. I'm assuming I should buy some whey protein to help?

To summarize everything since I went off on a couple tangents:

1. Skinny kid, vegetarian, tough to gain weight, looking to add 15-20 lbs in 4 months
2. Can someone offer a specific routine for me? (Type of exercise, reps, sets, etc...)
3. In terms of diet, what protein should I be looking to get, how many cals/day etc...

Thanks alot fellas!