How your body reacts to what you're doing depends on your body type as well. You may have to work a little extra to put on weight if you're an ectomorph. As far as routine, it's good to give a muscle group at least 24 hours and preferably 48 hours rest to let your muscles break down and regrow. From what I've read there are a couple of exceptions to this - calves and abs. You can supposedly work those every day and they'll be fine.
It's also very beneficial to work them out in primary/secondary pairs. For example, when you're working your pecs, your triceps are the secondary muscle group. The other is Back/Biceps. Shoulders get worked in with most upper-body exercises so those can be worked out on their own. Since legs are their own group altogether, shoulder's (which includes traps - i.e., shoulder shrugs) and legs are a good group to work together. Abs should be worked every day as a warm-up. If you put all that together you have yourself a 6-day workout:
Mon/Thu - Abs + Back/Biceps
Tue/Fri - Abs + Legs/Shoulders (this day sucks ass so it's good to put it in the middle of your workout routine b'c you're less apt to skip it if it's at the end or get a bad start to your workout week if it's at the beginning)
Wed/Sat - Abs + Chest/Triceps
Sunday - rest, no weights but cardio is very much okay
You should be eating a healthy amount of complex carbs within an hour after your workout too. I always did mashed potatoes, but sweet potatoes are even better. Whole grains are great too as long as you're not eating stuff that's too refined and/or contains a bunch of bad fat or other chemicals. After all, you want to put on weight in a healthy manner.
For muscle building, your basic workout should consist of 4 sets per exercise - 1 for warmup (50% of your max for that exercise at most) to get about 10-12 good reps, then raise the weight so that you're only about to do 8-10 good reps and do 3 more sets. Form is EXTREMELY important when lifting so do it right - it's not about how much you lift, but how you lift it. If you're unsure of the proper form for an exercise (regardless of whether it's a machine or free weight), get with a trainer or look up how to do the exercises on the internet. I really can't stress enough how important form is.
I would not recommend eating a big meal before you go to bed. Yes, it will put on weight, but it will also affect how well you sleep as well as how quickly you recover. Proper sleep is essential, so do yourself a favor and get it. That means not eating at least an hour before going to bed and getting a full night's rest.
Do not skip breakfast. Have a big, healthy one and like
Kelth said eat several meals throughout the day. Your caloric intake will depend on a few factors so just setting an arbitrary number usually isn't the best thing to do. There are a lot of sites that will help you figure out what your daily caloric needs are. I like
MyFitnessPal personally.
Keeping track of your calories isn't as important as it would be if you were trying to lose weight, but it's still a good idea b'c it will give you a picture of where your calories are coming from (fat/carbs/protein/sugar) so you can make adjustments as necessary. Again, you'll want to gain healthy weight for obvious reasons. You don't want to start clogging arteries, putting yourself at risk for diabetes, creating stress or increasing blood pressure, etc. Basically stay away from processed foods (chip/soda/snack aisles, boxed dinner aisle, deli meats, canned foods unless it's vege's w/o added salt, frozen meals in a bag, sweets, etc). Obviously there are a couple of exceptions to the rule, but not many -
KIND bars for example, love 'em!
Learn to read
nutritional labels! This may not seem like a big deal, but the better food you put into your body, the more you'll gain from your workouts. Figure out the quality of the fuel you're putting into your body and your body will thank you. When reading the actual ingredient list keep in mind that food manufacturers are required to list the ingredients from most to least. So if the first ingredient is sugar (or some sneakily-named sugar like sucrose, fructose, dextrose, etc) then that's telling you that there is more sugar in that product than any other ingredient.
If it's your thing, keeping a lifting log is really beneficial for motivation and to know when you're starting to hit a plateau so you can change things up. It's great to be able to refer back to your last weight used so you don't have to remember it, and to look back and see how far you've come since you started.
Inevitably, you'll begin to hit what is called a plateau. Basically you're doing everything you normally do, but are making very little progress. This is where most people lose their motivation. The key to this is completely changing up your routine. Get away from your current workout and do full-body circuit training 3-4 days a week with cardio days sprinked in for the other 2-3 days. You can also change from doing 8-10 reps with a high weight to 18-20 reps with a lower weight. Changing up the actual exercises you're doing can help too (for example, instead of doing bench presses do dumbbell presses). Basically anything to keep your body guessing. The plateau happens at different times for different people, but typically the 30 day mark is when most people start their plateau. So if you can switch things up every month, you should be good.
Eventually your body will get to a state of equilibrium. It's happy where it's at given the calories and exercise it's getting and wants to stay where it is. If you're changing things up and still not getting any gains/losses, this is typically the reason why. In this case your best bet is to adjust your calories. Obviously be careful when lowering calories. Your body needs a certain amount to function and to keep it out of "survival" mode. At some point you'll get to where you can't really do either (well, you can but you're getting into extremes at that point). That's okay, that's where you want to be. You're in pure maintenance mode from that point forward - congrats!
Finally, as always, go see a doc first to make sure you're in good enough health to work out, yadda yadda, insert additional disclaimer stuff, etc.
That's all I can remember from my personal training days. If any more pops into my head I'll toss it up here. Hope this helped!
