Depending on the character you wish to cosplay, you might find that you need to add color or thickness to your wig - especially if the wig fiber content is thin. While it is possible to dye the wig fibers of a light colored wig darker, it's impossible to add highlights (lighter color) to a darker wig without physically adding more hair.
For most, even thinking about messing around with a threaded needle on a newly purchased wig might sound intimidating. Many people commission others to do what they're afraid to do themselves, after all. But commissioning can be very expensive, so learning how to do this yourself will save you lots of money.