Looking to use my first DSLR?

So I just got my first job and I was gonna save up for a new camera, probably the Nikon D3100, but my mom told me that I could have her old camera that she used to use but doesn't anymore. If I were to buy a new lense, would that work as well?

Probably not.
Is her old camera digital?
Is it a DSLR?
A film SLR is a bad idea. It's just more expensive than a DSLR after you get enough pictures and have all the film developed!
I love my D3100 but I think there are key features I didn't know about that make me wish I had gotten a D5100. But do you really need a DSLR? Consider a P&S camera with fully manual features as a less expensive(at or less than half the price of a D3100) alternative.
If she is willing to give you a DSLR, then of course you should take that and just not buy a new camera for awhile. Any DSLR is going to be great.

You are not telling a vital piece of information. What is your Mom's camera? How can we tell you if a lens will work on the camera when you do not say what the camera is? Is her old camera also a 3100? If so, you certainly should not spend the money on another one. It is a very entry level camera, and that is probably why she no longer uses it. Just buy yourself the "kit" 18 - 55 lens that normally comes with the D3100 to get you started.

What camera is she giving you?

Where is the soul?

DSLR Tthousands of photos digital thousands of photos insanity huge money. Then die.slr not huge money a few great photos then die.wheres the logic? And all the learning and all the questions… If there was still only film how many questions would actually get asked in this section GOD? WHY DO YOU THINK DIGITAL IS LESS EXPENSIVE THAN FILM FOR?

We can't answer that question until we know what sort of camera your mom has.