The term “tune-up” is essentially meaningless, and if you asked 10 different mechanics what a “tune-up” entails, you would likely get at least 8 different answers.
What you need to do is to consult the manufacturer’s Maintenance Schedule (it is in one of those booklets sitting in your glove compartment) for the appropriate maintenance procedures for your current mileage as well as for all of the mileage and/or elapsed time intervals that you may have skipped along the way. Once you have a list of what the vehicle’s manufacturer requires, then you can copy that list and shop around for price quotes on those procedures.
How will it help? Well, depending on how far behind your car is on maintenance procedures, servicing it properly should extend the life of the vehicle to some extent, and may improve both gas mileage and engine performance. (Note: If the vehicle is very far behind on proper maintenance, nothing will extend its life.)
Unfortunately, given the lack of information that you have provided, nobody can give you anything more than general information. If you want a substantive response, you need to post the following information regarding your vehicle:
Make
Model
Model Year
Odometer Mileage
Engine Type/Size
Manual or Automatic Transmission
AWD or not AWD
The date and odometer mileage of the last “major” service
How many miles you drive each day, and each year
Whether the timing belt (if so equipped) was ever replaced
Once you fill in all of this vital information, you can get some responses that are more specific.