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Hi,
With a Newtonian telescope at a minimum you need to collimated it every time you take it out, if you want it to perform at it's best and give sharp images and pinpoint stars. Over a long nights observing I might recheck my collimation a couple of times and make some very minor adjustments.
The deluxe Orion collimating tool Jarrod has pointed you to is the best place to get started. It enables you to check "everything" in the collimation process and end up with an accurate result. Some people will suggest a laser device. This isn't what you need as it does not facilitate checking the positioning of the secondary mirror and can in some circumstances give an inaccurate result.
Cheers
John B
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