First off. let me just say that the following you are about to read is aimed mainly for the skinny, underweight type person looking to add mass to their frame. So, someone looking to drop weight for the summer would do things quite a bit differently as this program wont really work for you.
The equation is simple. lift weights + eat food = muscle. sounds easy right? well not to everyone. unless every aspect of the forementioned is dialed in correctly you will not change your body too much. let me explain.
Firstly ( is that a word? ) if your skinny and looking to add size the number one thing THE #1 !!!! thing to do is EAT!!! i dont mean just junk food i mean healthy foods packed with the right kind of calories, carbs, and protein to add the muscle mass that you work so hard to achieve, to your frame.
the genral rule of thumb, lets say for someone 160 lbs, i suggest eating 3200 calories, 280g of carbs, and atleast 210g of protein a day. ill show you how to do that in a little bit.
lets start off with the fun part, weight training!!!!!
WHat people dont realise though, is that if you follow most workout routines in bodybuilding magazines you will most likely not gain much size, and may even lose some!! this is caused by overtraining. as the skinny person with a fast metabolsim, you have a limited recovery ability and anything over the amount of stimulous needed to build muscle will start to turn your body into a catabolic state ( not good. ) so this is the workout program i recomend:
Day 1
bench press - 4 sets of 8, 6, 6, and 4 reps for each set, followed immediately by a superset ( superset means to do the following exercise right after the last set only of the previous exercise ) of lying dumbell flyes.
Military press - 4 sets of 8, 6, 6, and 4 reps with a superset of dumbell laterals
lying tricep press - 4 sets of 8, 6, 6 and 4 reps with a final superset of overhead dumbel presses
Day 3
bentover rows or cable rows - 4 sets of 8, 6, 6, and 4 reps with a superset of pullups
barbel curls - 4 sets of 8, 6, 6, and 4 reps with a superset of incline curls or concentration curls
dumbel wrist curl - i like to do 3 sets of atleast 20 reps to really torch the forearm muscles!
Day 5
SQUATS!!! ( do these with PERFECT form or you could really injure yourself ) start with 12 rep squats, then when youve mastered your form, start do the 8, 6, 6, 4 rep scheme with a superset of leg extensions.
For the hammys ( or leg biceps ) there is a great exercise to do for them but until you are atleast an advanced bodybuilder i dont recommend doing them as doing them unproperly will DESTROY your lower back, so just do 4 sets of 8, 6, 6, and 4 reps of lying leg curls.
* and for calves i recommend doing 2 sets of as much weight for 20 reps as you can, the do 2 sets of 6 reps to really burn your calves as they are the most muscle fiber dense area of your body.
REMEBER TO ADD A LITTLE WEIGHT (MAYBE 5 LBS) TO YOUR WORKOUTS EVERY WEEK!!!
this routine looks easy on paper but it is very brutal. it focuses on mainly compound classic bodybuiling movements ( bench press, squats etc ) that stimulate the most amount of muscle fibers in the shortest amount of time. remember this:
muscles are stimulated in the gym, BUT THEY GROW WHILE YOU REST AND EAT!!!
i suggest weight training in the 8-4 rep range as that stimulates the most amount of muscle fibers. 10+ reps are mostly for endurence, 3 reps and less are for strength, but 8 -4 are for strength AND size!
which brings me to my next point, food.
IMO eating is 90% of the battle of gaining size. the body needs feul to rebuild and repair itself ( grow muscle ). since most people's schedules and food habits vary, i wont write down an exact eating schedeule, but you must, and i mean MUST consume the right amount of food in the right quantities. this means instead of eating as much food as you can in 2 or 3 sittings, you must eat 5 -6 meals EVERYDAY!!!! strive to eat 500 calories and 40 g of protein at EVERY MEAL!!!! like i said, EATING IS 90% of the battle!
sample of "superfoods" to help you gain: chicken, beef, eggs, whey protein, milk, pastas, oats..etc. if your busy pack a rubbermaid tub of a chicken breast with some rise, or have a protein bar EVERY 3 HOURS. YOU MUST EAT EVERY 3 HOURS!!!! seeing a pattern here?..
next subject, supplements. there are many supplements on the market that do little more than empty your wallet. there are a few supps that i recommend however that will help your bodybuilding endeavers. an MRP ( meal replacement powder ) can be quickly and easily made at any time of day and counts as a whole meal. for example you could eat 4 whole food meals of chicken yadda yadda etc a day and have 2 servings of MRPs a day and meet your quota. another supplement i recommend is whey protein. whey is the best protein you can get, but has little or no carbs, so for one meal you could have whey protein and some rice or pasta for carbs, and boom, thats one solid, muscle building meal! to top it off, i recommend a multivitamin everyday ( for men make sure it is WITHOUT iron ). any regular multi vitamin will do. if your body is missing any trace vitamin or mineral it wont grow optimally! also for pre workout, i recommend a whey drink and some carbs ( bread, rice ... whatever ) and post workout **!!!!( another secret!!! right after you work out your body is STARVING for nutrients so during this 1 hour post workout window, if you feed your body right it will treat you right

) so i recommend whey, some chicken for extra protein, and plenty of rice or bread for carbs IMMEDIATELY after you workout.
Im sure theres more to add but my brain hurts right now from writing all that so im done for now. if you have any questions, please ask away. this is the first step to helping you get the body of your dreams, all you have to do is follow my guidelines, and you will be on the fast lane to muscle growth!!!! good luck.
P.S. one more thing. the #1 secret to muscle gains is ..... CONSISTENCY!!!! keep at it!!!