The starter is not in the easiest of places, but it's a job that you can do yourself with the right tools and a little bit of patience. The starter on your 2008 G35x sedan is located on the passenger side of the engine, right on the bell housing of the transmission. It's a bit of a tight squeeze to get to, and you will need to get under the car.
Here's a general guide on how to get it out:
First, you should disconnect the negative battery cable to avoid any electrical issues. Next, you will need to get under the car and remove the underbody splash shield. This will give you a better view of the starter.
The starter itself is held in by two bolts. You will need a socket wrench with an extension and a universal joint to get to them. Once you have the bolts out, you will need to disconnect the electrical connector and the main power wire from the starter.
The starter can sometimes be a bit difficult to get out because it can be seized to the bell housing. You may need to use a pry bar or a hammer to gently tap it and get it to release.
There are many detailed videos on YouTube that will show you exactly how to do this. You should look for a video for a 2008 Infiniti G35 or a G37, as they are very similar. Watching a video will give you a better idea of what to expect and what tools you will need.
It's a very common job, so you should be able to get it done without too much trouble.