How to know what wheels to buy to widen stance?

I have a '97 Geo Metro and want to buy new wheels for it.
What is offset, and how does it work?
How do I measure the existing offset?
How do I go about knowing what offset to look for in order to widen a vehicle's stance?
Thanks a lot.

well more than anything, wider wheels would do the trick.
wider tires would help too.

what is the tire size on your geo?
they are probably around 185. upgrading to around 205 or so would help some.

https://www.rsracing.com/tech-wheel.html

Read this. It explains offset.
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101

Widening the stance is probably not an option for you, at least not more than a half inch or so. Take a look at how close your current front tires are from the fender. If you widen the track and you're making a turn at 35 mph and hit a bump, you run the risk of blowing the tire and destroying the fender if they touch.

Careful fitment is required to get it right. I suggest drawing on the experience of other geo owners who have done different offsets and tire sizes first so you can be assured that a combo will work.

Great links, thanks.