Since you have a 42", especially if its newer you could just use toggle bolts on the drywall (because 42" plasmas are very light weight) What i personally use in my installations are (http://www.toggler.com/index.html
) but any toggle bolt with the proper weight requirements will do. If you don't feel comfortable with that you just need to use the toggle bolts with the stud. Most brackets are going to support your 42" with ease. The most common mistake is people using a level that comes with the bracket, they are usually not level. If you don't feel comfortable or are unsure don't do it. Have a trusted professional.
Here is a link to some solid installation instructions.