Help my fuel gauge isn't working. Lexus..?

I drive an 1996 Lexus es300. Its my 1st car and I am an teenager. I just started driving two weeks ago. I brought my car from an autowhole sale. So I been driving my car around for like two weeks. And my fuel barely goes down And somtimes it goes back up. to fill tank randomly. And I ask my dad and he said well you do not drive lots and you do not use ac lots so your fuel should not be used that fast. And I am thinking o wow this Lexus is fuel economical. But the other day while I am driving my car the fuel empty lights came on and I was so perplexed bcuz my fuel meter is like full. Then I went to put some gas on it. What am I supposed to do? Is it costly to pay? I do not have much money? I called the place where I got it from and they said theres nothing they can do about it because my 30 day warrenty does not cover it. It only covers the engine. That does not make any sense. HELP. THANKS

it is a '96 stuff breaks. do not waste your money fixing it.

Look in the manual for the gas tank size and keep track of your fuel economy when you fill up. Do some simple math and set your trip odometer every time you fill up. make sure to top off again well before reach your magic number.