tune up?

hey guys. im lookin at gettin a tune up on my car cause im gettin crappy gas mileage as some of u may kno...its actually gone up to around 16 mpg now...but what is involved in a tune up? what would they do and how much would it cost? i need new spark plugs so would they just inspect them or would they put new ones in? can i just get the low-down on tunups?

thanks

spark plugs, wires, air filter, fuel filter, distributor cap and rotor, change oil/filter

:iamwithst

Also grease where there are zerk fittings. (ie: balljoint).