Well, you have to get a light! The commonest way is to find a stick (they lie around in forests), go to a fire (all the main buildings have one or more) and LIGHT STICK FROM FIRE. You will then be able to see in the dark. Sticks are called 'brands' by most people (although to be pedantic they only become that when they're alight!). WARNING: entering the swamp carrying a lit brand is fatal! DOUSE BRAND before you go in, or just DROP BRAND nearby. You can use lit brands to burn down most doors, but the brand is also consumed in the ensuing blaze in such cases.
The second way to see in the dark is by having someone cast a GLOW spell on you. You will often hear people shouting asking for a glow, because they can't cast the spells themselves. Magic-users can, however, and since it is a "cheap" spell to cast, will often oblige if you ask politely. If you are obviously a complete novice, stumbling around lost in the dark, you may even be given an unsolicited glow by a wizard or witch who has noticed your plight. It's always polite to shout thanks if that happens.
You can get to glow yourself if you find the circle in the attic of the inn, and drop some treasure in it. It will then 'tingle with magic', and you can ENTER CIRCLE to start glowing. Certain objects glow in the dark, eg. some swords, but if you do find such an object, always keep a check on your stamina...