Intro: Quickly Brightening Up a Photo in Photoshop.
You could skip the reading and watch the video. That's in real time, it's not a complicated or long process... Turning on high quality and full screening helps...
Step 1: Open Up, Levels.
So open your image.
Now go to your levels.
You'll see a little graph appear, first off bring the two end sliders up to where it starts on either side, it'll probably start looking brighter already. Now move the middle slider around to get a feel for brightness, choose the spot you like the look of, ignore the colouring for now.
If you're having trouble with very dark or very bright spots you can use the lower slider to cap off the highest and lowest levels by moving the sliders on the little gradient bar below. Try to avoid doing this though as you'll lose colour information as well, this can make an image look very washed out if you do it excessively.
Step 2: Duplicate Your Background Layer.
Right click on your background layer and click duplicate layer, alternatively you can drag it to the the little new layer box, either works. There's no native short cut for this, that's annoying sometimes, I added one (ctrl+/) which is nice and handy for me.
Set the mode of the duplicate layer to colour. To do this make sure you have the right layer selected, if you're not sure click on it, it should be highlighted. Click on the drop down box t the top of the layers palette and down near the bottom of the list you'll see colour.
Step 3: Hue/Saturation
Now to improve the saturation a bit.
Go to Hue saturation - Image>Adjustments>Hue/saturation (Ctrl+U)
Slide the saturation bar up and down a little and watch the image, now bring it up to a level that looks good, a little blockiness is OK but try to avoid it, look for when the image looks best to you. Now hit OK.
If you want you can try playing with the hue bar to shift the colours, a small amount can make it look like a different light, some interesting effects can be had with stronger doses though.
Step 4: Blur the Colour Layer.
Now we put a bit of blur on the colour layer to remove any blockiness and smooth out the colouring, too much will cause objects to have colour in the middle and none around the outsides, which is a cool effect in some cases but not exactly what we're going for.
To get the gaussian blur up it's Filters>blur>gaussian blur.
Move the slider up and down and watch the image being worked on, not the little previewer box, it's only showing you what that layer would look like.
A blur of around 3-6 pixels will probably be about right, go down to two for small images. Larger images should be OK but always check by looking at the result, if you ever have trouble on looking at the effect then apply it and hit ctrl+z to undo and again to redo so you can see whether it's what you want, especially helpful when you're halfway through a big stack of images.
Step 5: Brightness/contrast.
Your image might already look good enough for you, however some still need a little help with the contrast and brightness.
So duplicate the background layer and set the new copy to luminosity, open up brightness/contrast box by going image>adjustments>brightness/contrast
Now lift the contrast and brightness slowly until you get a good level of brightness.
The reason we do this on a luminosity layer is if you try to modify it on a normal colour layer the colours go mad and very blocky.
It's OK to go a little overboard on this one, mainly because fiddling with the layer's opacity a little will tend to be the easiest way to fine tune the result. If you're having trouble with one end being too bright or dark after the change you can do a variety of things to fix it. For small details use the burn tool to lower bright spots and the dodge tool to lighten up little dark patches, set the exposure down low though, otherwise it's hard to control the result smoothly. For larger bits consider using a gradient mask to make it go from transparent to opaque at the right bits. Another gradient approach to give a smooth bright to dark across the image would be to make a luminosity layer and put a gradient across it, then play with the opacity to get it right. Using a black and whites with greys in between tends to work best.
Step 6: Check Out Your Results and Decide It You Like Them.
I've popped a little comparison image together there to give an idea of the difference, this isn't a perfect way of doing things but it gives you a set of basic steps to making an image look a bit better if it was too dull to start with.
You'll notice I've not bothered with noise reduction here, one plugin that's good for noise reduction is neatimage, photoshop's built in noise removal tools are alright but there are better for free, I'll dig up a few, it's not a complicated process, in fact many of them work really well on their best guess settings.