The key to building muscle is to use heavier weights and do lower amounts of repetitions. Lifting heavier weights will cause your muscle micro-traumas which as your body heals will cause your muscle to grow and leave you with bigger, stronger muscles. Here are a few exercises you can try
Step 1: Bench Press
There are several things you need to remember to stay safe when performing a bench press. Firstly you should always have a 'spotter', in case the weight gets too much or something goes wrong, they can replace the bar onto the stand and keep you free from harm. Bring the bar down to mid chest height making sure you keep your wrists straight. When you raise the bar again do not lock your elbows at the top of the lift.
Step 2: Upright Rows
Make sure you have a firm comfortable solid footing with feet shoulder width apart. Using an over hand grip bring the bar up to chest height. Your hands should also be shoulder width apart on the bar. Keep your elbows above the bar at all times and your back straight.
Step 3: Bicep Curls
The first rule to remember when performing a bicep curl is DO NOT SWING. Your whole body should be steady and strong as you do them. At the start of the curl your palm should face upwards and then you can bring the weight up to your shoulder. If you feel more comfortable you can always lean you elbow on something for stability.
Step 4: Tricep Extensions
When performing a skull crusher you are laying on your back and curling the weight down towards your face. It is for this reason you should ALWAYS have someone to spot you so you don't end up with a broken nose if you drop the bar. Your palms should be shoulder width apart on the bar and you must keep your wrists straight throughout the action. Your elbows will finish pointing toward the ceiling. This will work on toning up the backs of your arms.
Step 5: Squats
When performing a squat always make sure you keep your back straight and your tummy in. Your weight should be on your heels and as you push your hips back you start to bend your knees. Your knees should not come over your toes. Try doing sets of ten squats to start off with. You can increase this the more confident you get. To make it harder put some weights on a bar across your shoulders.
Step 6: Pull Ups
The area of your back you want to work on depends on your grip on the bar, a wider grip will work your lateral muscles where as a narrower grip will work the middle of your back. Please see VideoJug's "How to do Pull Ups" for the correct technique.
Step 7: Diet
Eating more protein in your diet is a simple and effective way of building muscle. Protein is made of amino acids which are the building blocks of muscle. It should not substitute all other vitamins and minerals but try eating more brown breads, nuts, seeds and chicken.