First of all, to be clear, there are two types of darkness you need to worry about. Regular darkness and then that supernatural stuff that is usually only possible in the area of a deeper darkness spell. A simple torch, lantern, sunrod, or light spell generally suffices in regular darkness, though it's a beacon that gives away your location to every foe. However, when in the area of a darkness spell, these simple solutions will no longer work.
Against areas of darkness where light won't work, the easiest thing to do is to just get Darkvision. Now, dwarves, half-orcs, and many others already have it. In that case, you don't have to do anything, just fight normally. If not, your main easy options to gain darkvision are the darkvision spell, which lasts for hours, even on a scroll (150 gp or 2 prestige for 5) or potion (300 gp), or for a shorter duration that also provides some ancillary benefits, alter self (150 gp for a scroll, potions not possible).
You can also use a light spell of a higher spell level. Continual flame cast by your party cleric will do, but if you got one from anyone else (or an ioun torch), it won't work, as the lower-cost version used for your purchased casting is not strong enough to beat darkness. Daylight is a bit confusing, so I'm going to give it its own section below.
So what if the area is supernaturally dark and even Darkvision is useless? There's a few ways to proceed. The most expensive choice, unless you have an alchemist with alchemical allocation, is the Elixir of Darksight. At 1200 gp a drink, it will upgrade your Darkvision to see through supernatural darkness for one hour. If you happen to possess the blood of fiends within your veins, you could also train your Fiend Sight twice in order to be able to see out to any range in any level of darkness, supernatural or not.
The best solution to help the whole group is to bring out a heightened continual flame heightened to 4th level or higher. This will shine brightly and annihilate the deeper darkness completely in its range. Lasair, who is teaching the lessons on the major deities of Golarion, can give you a heightened continual flame if you adventure with her.
So what about daylight? It has a huge range, but it acts weirdly when combined with darkness magic. In an area where daylight overlaps with any darkness spell, the magic is cancelled completely. So bring Darkvision or a mundane light and you can see just fine. This is true for darkness magic of any level, 0 through 9. Because it is readily available for 2 prestige, oil of daylight is a mainstay of many adventuring groups (you could get a scroll with two castings for the same price, but then you might not be able to read it). There's also a weird thing you can do where you go up to the center of the deeper darkness and directly apply daylight via touch or slathering the oil on it. This automatically dispels the deeper darkness, but many enemies that have the ability, like dark stalkers and drow nobles, can cast it all day long, so this is usually a terrible option (and incidentally, you'll want to keep them away from poking your daylight for the same reason. You can also counterspell deeper darkness with daylight, but since it's also via touch, something very weird must have happened for that to be an option, since the enemy can just move away first.
So what if the enemy has you outclassed? They dispelled your daylight and are sneak attacking your team in the dark. You need to even the odds as best you can on the cheap, and there's no choice better than a simple obscuring mist to do that. It will negate sneak attacks, and it prevents the enemy from aiming any better than you can unless they go right up next to you (thus allowing you to at least retaliate). Obviously this doesn't work if the enemy has large area attacks, like a nightshade. Just like with invisibility, training in Blind-Fighting is not a bad way to even the odds either. Combined with obsuring mist, a trained blind-fighter is only barely less able than an enemy that can actually see in the darkness.
Now that you've learned invisibility and darkness, we've covered two of the main ways that enemies turn a battlefield into a killzone and how you can stop it. Join me next time and we'll cover another.