Ah, yes, the elusive G spot. Some women have so many problems finding theirs that they consider it mythical.
We now know that the G spot is somewhere on the upper wall of the vagina, an inch or two past the vaginal opening. Which means that many men power their way past it in search of deeper penetration. Sorry, guys. It's easier, in fact, to find with fingers - which some women are not comfortable with (consider using lube) and often easier if the woman lifts her knees towards her chest. Not every woman gets really great G spot orgasms, but they are quite something for those who do. Using fingers to locate it is good for guys and lesbians alike.
As a guy, if you even know what the G spot is, you're ahead of most men (because sex education sucks in most places).
When it comes to finding your own G spot, use fingers or a toy. G spot vibrators are designed for this - but again, they don't work for everyone. The G spot is a definite physical thing, however. It's a little bit ridged or "rough" unlike the smoothness of the rest of the vaginal wall. Oh, and G spot stimulation does make some women squirt - it's now theorized that the G spot is what the prostate gland becomes if the body develops female.
Lying on your back with your knees up is definitely the best position. Fingers are easier to find it, but once you have it located, a toy can give much more stimulation and a much more intense orgasm. When with your partner, ask to go on top, because that puts the penis against the upper wall. Doggy style also works well to make it easier to find during penetration. Or, you know, just use toys.
Kegel exercises also make it easier to find the G spot - and can improve sex for both you and your partner both. Rhythmically clench and unclench the muscles you use to hold your urine. Doing these muscular contractions during intercourse will help bring his penis against your G-spot and it will make your vagina feel "tighter," which really makes things good for the guy too.
So, if you've found your partner's G spot, what should you do with it? Rhythmic or circular stroking with a fingertip or "tapping" - touching it repeatedly - can both be very effective. You can't generally get there with your tongue, though, but you can always combine cunnilingus with fingering or using a G spot vibrator.
Regardless of how you do it - finding the G spot can make for incredible orgasms.