Really a three stage process:
Need to properly diagnose the problem-- could be something as simple as loose or broken wire (if an external sense set up, that small wire is particularly susceptible to issues).
If you do need a new alternator, it is CRITICAL that you determine the proper replacement. Most are different than truck alternators that may look the same.
All will have a B+ terminal. Some will have a ground lug. Some will have an external sense terminal. Some will have an excite (ignition hot powered)terminal.
Go back with EXACTLY what you have in terms of connections!
Last step is to install the new one.
How much of this you choose to do/hire out is up to you, but you need to be aware of the necessary steps. Many times truck techs install an incorrect alternator for our application. If in doubt, call your chassis maker for exact alternator make and model.