Bows and crossbows are specialised two-handed weapons. You should first swap them to your off-hand (see ‘help swap’), and then assist with your other hand (see ‘help assist’). This is because you won’t be using the hand holding the bow - in archery, you use your primary hand to draw the string.
Make sure you are wearing a quiver (or a case of bolts if using a crossbow) and then use the load command with your assisting hand (i.e., ‘ll’ for left load, or ‘rl’ for right load, depending on which hand is assisting). You will have to reload again after you fire.
If you are using elementally enhanced arrows or crossbow bolts, you can use the charge command to charge them up before firing. Non-charged arrows and bolts perform burst attacks on a critical hit, while charged arrows and bolts only need to successfully hit your opponent in order to perform a burst attack.
Arrows and crossbow bolts can be blocked with shields, deflected with magical defences such as forcefields, or dodged. A dodge only has half its normal defence when used against ranged weapons, however. Certain talents, such as Expert Marksman, can help make bows a more viable choice.