ok im a girl and i need some help

ok so im new here i just bought a 98 tiburon just a few weeks ago and already im having problems, apparently with the transmission not shifting properly it is an automatic , but ive had conflicting opinions and some people think its the 02 sensor...before my ex and i broke up i used to drive an RX8 and i pretty well only tinkered with rotory engines... so im just starting to get used to having a "normal' car, has anyone else had this problem?

Is the check engine light on?

come on and off randomly

I would check to see what codes are there to see if that gives you any insight as to what is happening.

i did, it said it was the tranny not shifting, how much does it cost to fix or do i have to replace it?

What codes specifically did it give?

How many miles are on the car?
When you say not shifting, what gear? Can you pop it into 2 or 3 and see if it shifts between gears or does it only occur in Drive
Who was the previous owner and did he beat on it?
Since it's an auto tranny check to see what kind of color the fluid is, is it dirty? is it excessively dirty?