Prepare weights corresponding with your fitness level and type of training. Take a stance with your feet shoulder-width apart. Grab the barbell so that the palms of your hands will be facing forwards (radial sides face outward, supination) at a shoulder-width grip.
Lift the barbell up all the way to shoulder level by flexing your elbows. Keep your elbows close to your torso. Subsequently, go back to the starting position in the same manner. As you go up, breathe out and as you go down, breathe in.
Make sure you have the right stance and barbell grip.
Avoid letting the bar drift too far forward and excessive dorsal flexion in your wrists.
Control your stance and avoid bending backwards and overarching your lordosis.
To make sure you are making the correct motion, watch yourself in the mirror.
To improve control over your motion and body stance, it is advisable to start doing this exercise with your back leaning against the wall to help prevent excessive elbow motions as well as swinging.
Barbell curl is a basic biceps exercise. Its main disadvantage is the unnatural position of the wrist that can cause problems especially with higher weights.
This is why many athletes prefer EZ-bar over straight bar.
The closer grip will strengthen and build more the outer part of biceps muscle (the long head of biceps brachii and brachialis muscle) while the wide grip is better for the inner (short) head of biceps brachii.
Do not cheat in first repetitions. Try to do the movement slowly; you can apply some cheating in the last two reps.