ive always been naturally slender, but ive also been working out and doing martial arts for many years.
for me, what really had the most profound effect on my body was deadlifts, squats, farmers walks and variations of crossfit thrusters/clean and jerk
dont waste your time with isolation exercises right now, do the exercises that require the use of the most muscles at once. why spend an hour doing chest or arms only when you can work out your entire body.
also, if you are slender or a hardgainer, working out too much will leave you overtrained and you will lose too many calories. you need more rest, especially if you are taxing your body with full body lifts. once or twice a week is all you need. make the deadlifts and squats your main exercises, everything else supplemental.
ive been having good results lately with:
squats 3x10 and 2x3
deadlifts 3x10 and 2x3
thrusters 4x 4
chest and back super set with machines(chest press and rows or flys and rear delt machine) 4x10 each
dip machine 4x10
abs
calves
farmers walks
this takes me 1 hour to do, it may be too much for a beginner
here is a hot chick showing how to do thrusters, a great full body movement
http://www.youtube.com/watch?v=XfIc4DUeapo
also i dont do bench press, i dont want to deal with a bar loaded with weight falling on my face when i can work out my chest perfectly fine with machines, pushups and cables, cut it out to save time and eliminate risk of injury, use a seated chest press machine instead