First, replace the missing spokes with properly sized spokes.
Next, on the drive side of a rear wheel with derailleur gears you will need to remove the cassette, insert the new spoke(s), then replace it.
That requires a special tool (there are several variations for different cassettes and freewheels).
A bike shop can do this or sell you the correct tool.
You will always need the next special tool: a spoke wrench that fits the size of spokes you have.
While a truing stand is great, you can do some basic truing of a bike with rim brake by simply putting the bike on a stand.
With disk brakes a cable tie round the bike frame or fork can be turned so the free end is almost (but not quite) touching the rim.
Anything similar will work, you just need something close to the rim that lets you see the gap between that and the rim.
Let the air out of the tire.
Air pressure in the tire will push the tube against the turning top of the spoke head, and can puncture the tube.
Spin the wheel slowly and watch the space between the rim and the brake pad.
When you have found the centre of a an area that is listing to one side, use your truing wrench to loosen the spoke nearest to it on that side ¼
to ½ turn, and tighten the spokes on the opposite side an equivalent amount.
As long as you work in small steps and work slowly, it is safe to play around and figure out how truing works.
Patience is required.
If the rim itself is bent, it may be hard or impossible to true.
bicycles.stackexchange.com/users/17/Drew Stephens
As per the other responses, the main tool you need is a spoke wrench.
You can use the frame itself as a truing stand.
Other things to note are: Deflate the tyre before performing the re-truing.
If you don't do this, you can end up drilling a hole through the rim tape with your spoke nipple.
Look at the way the other spokes are laced to get the lacing of your new spokes correct.
Consider buying a few extra spokes from your local bike store when you get the replacements.
I find that once a wheel reaches the stage where a spoke or two breaks, it is common for a few other spokes to break not long after.
bicycles.stackexchange.com/users/350/deemar
The basic tool you would need is a spoke wrench .
This will allow you to pull the rim (by the spokes) back into true.
The spoke wrench is actually turning the spoke nipple out at the rim.
The tool you would need in order to tell if it is true is a truing stand .
The tool you will need to make it all work is your brain.
=-]
bicycles.stackexchange.com/users/50/Jack M.
For your wheel, you'll want to get spokes cut to the length of your wheel by a local bike shop.
The length depends on the rim size, the hub size, and the spoke pattern, so the best way to do this is to bring the wheel in yourself.
Bringing the wheel in will also help the mechanics to tell you if it is beyond repair - once wheels have bent more than an inch or so, they rarely can be trued to be as strong as they were.
Once you have the spokes and the nipples to attach them to the rim, you can tighten them with a spoke wrench.
Once you have the new spokes tightened down, work your way around the hub, tightening spokes to adjust the rim position.
Keep in mind that you are tightening the spoke into the nipple, so if you are looking from hub to rim, you want to turn left to tighten and right to loosen.
There's a more in depth explanation of this up on wikibooks, as well: http://en.wikibooks.org/wiki/Bicycles/Maintenance_and_Repair/Wheels_and_Tires/Truing_a_bicycle_wheel
bicycles.stackexchange.com/users/367/Dan Monego
Yet another very-slightly different answer follows below.
I never owned a stand, and don't have the space for one in my apartment.
Plus I prefer to do it in the park in clear air with a beer in hand.
My procedure: (0)
This answer concerns only the in-frame method, that is, when you have V-brakes.
Why would you true a wheel on which the rim is not rubbing against any pads?
(1) Turn the bike upside down.
(2) Check if there are any very loose or very tight spokes.
Your wheel is already out of true, it won't hurt to correct problems.
Overtightened spokes may break, while loose ones might rattle when they become even more loose.
(3)Tighten your brakes excessively.
That is, unscrew this so that the wheel barely turns and rubs most of the time: (4) Turn the tire and find the place where it rubs the most.
Identify which side it rubs and correct it as others have noted.
(5) Repeat 4.
lots of times.
(6) Finish your beer and get another.
bicycles.stackexchange.com/users/3927/Vorac
I broke a couple of spokes on my wheel, now it's all bent, how can I align it back again? What kind of tools do I need to do a good job?
How do I align a wheel?
bicycles.stackexchange.com
<wheels><repair><spokes><wheel-truing>
bicycles.stackexchange.com/questions/186
bicycles.stackexchange.com/users/29/Johan Dahlin
S0: The basic tool needed to balance a bike wheel is a  spoke wrench .   Turn the bike upside down. Check if there are any very loose or very tight spokes. Tighten your brakes excessively. Unscrew this so that the wheel barely turns and rubs most of the time. Turn the tire and find the place where it rubs the most. Identify which side it rubs and correct it.  The basic steps to align your wheel are the following. First, you should get some replacement and spare spokes at the length of your wheel. Turn the frame upside down, deflate the tires and remove the cassette. Attach the spokes to the rim, tight them with a spoke wrench and perform the re-truing with necessary adjustments. Most testimonies agree that the tool necessary is a spoke wench,
The basic steps to align your wheel are the following. First, you should get some replacement and spare spokes at the length of your wheel. Turn the frame upside down, deflate the tires and remove the cassette. Attach the spokes to the rim, tight them with a spoke wrench and perform the re-truing with necessary adjustments. Most testimonies agree that the tool necessary is a spoke wench,