All the serpentine belts crack. Some of them crack within the year. If there
a segments missing from the belt then the belt needs to be replaced. As far
as he tensioner  goes, its self adjusting, look at it with the motor running
and it should be moving slightly back and forth. If its doing that, its
working as it should, if it is not moving it is seized up.
Changing the fuel filter is a preventative maintenance and maybe should be
replaced every 5 years, but if you are buying gasoline from a reputable
service station you get good gas.

These two item are the biggest "get yah" things that some garages pull on
people.
i took my 1997 malibu in for an oil change and they told me my serpentine
belt is cracked and the serpentine belt tensioner needs adjusted it wasn't
moving for them is this a common problem i had my belt replaced probably
about 4 years ago i have 151,000 on it.. and need it to last as long as
possible.. they were going to charge me $192.90 they also said for general
maintenance i should have my fuel filter replaced also.  thanks for any
suggestions

Belts are a Maintenance item, like brakes and tires, and need to be changed.
Fuel filters are the same as oil filters, they need to be changed on a
regular basis.
READ the owners manual, it will tell you the recommended intervals and what
to have serviced  on the car. At 151k a lot of things are gonna start to be
worn out.