I'm a naturally tall lean guy so, I lift for power and to gain lean muscle fast. This sounds like what you want to do. I've been lifting around 4 years and 2 of them more serious. I read a lot about weight lifting and I've found some good tips for making your workouts much more effective.
1. LIFT HEAVY. You want to warm up good before a workout to decrease the risk of injuries but, durring your routine lift weight that you have a hard time doing about 6-8 reps with. A partner is great for this because you will want a spot. If you don't have one, use dumbells.
2. EAT. Within 30 min of your workout you should eat a large amount of protein and carbs. After a workout your body has a temporary increase in insulin production that will make this the most important time of the day to feed your muscles. Diet is a huge part of making your workouts effective. You can lift all you want but if your body doesn't have protien and carbs you will not gain any muscle. Try and eat at least 1g of protein for every pound of body weight. ie 115lbs minimum 115g of protein daily. Eating tuna (chunk light is some nasty shit) or buying protein shakes is a great way to do this. 1 can bumblebee solid white albacore has 37.5g protien.
I'm not sure what your routine is but I normaly work out 5 times a week for 45-60min. This lets me push my muscles hard and then gives them a whole week to rest. I used to be exactly like you though. I lifted and never got much stronger for a long time but the I started reading and changing my schedule. Now I get MUCH b***** gains in a lot shorter time. Mainly because of my diet and routine. If you are curious here is my routine.
Mon.- Chest Tues.- Back Weds.- Shoulders Thurs.- Biceps Fri.- Triceps, Legs
Though its not quite as effective if you don't want to workout 5 times a week do this.
Day1- Chest, Biceps Day2-Back, Triceps Day3- Shoulders, Legs
In the last 2 years I went from 157 lbs to 183 lbs and dropped body fat %. This may not sound like too much but I've had to take a lot of time off in those 2 years. Broke my arm 3 months. Crushed my scull another month. Lots trasportation and moved 5 months. To keep fat off I run 3 times a week and bycicle. Hope you get somethign out of this.