Regardless of whether you're looking to use retrofit or ballast compatible LED tubes in your existing fluorescent fixtures, you'll need to know what kind of ballast your fixture has.
For retrofit LED tubes, you'll be re-wiring your fixture to bypass the existing ballast, and just how that re-wiring is to be done depends on the type of ballast you have.
On the other hand, if you're using ballast compatible or "plug and play" LED tubes, you have ensure that the tubes you're buying are compatible with the type of ballast your fixtures have.
To help you out, ELEDLights.com has made a handy diagram.
For those of you using retrofit LED tubes, you'll want to note that, with fixtures that have instant-start ballasts, you'll need to replace the G13 socket (also known as "tombstone" or "lampholder") nearest where the power comes into the fixture to a non-shunted G13 socket. Don't worry: the tombstone replacements, like the ballast rewiring in the first place, is a quick, simple task.
You can find 2ft, 4ft, and 8ft LED fluorescent replacements at ELEDLights.com—and if you want to replace your entire fixture instead of worrying about the ballast, we carry LED-ready linear fixtures too!