To make her squirt, you most likely have to use your fingers on her g-spot. Few girls can squirt from penile penetration alone.
To find the g-spot, get her on her back, stick your index finger in her and make a "come her" movement. You should be able to feel your finger rubbing against a "rough patch" that feels a bit like sandpaper. That's the g-spot area. When she gets horny things will start to swell up in there, making it easier to locate.
What you must do is to fingerfuck the shit out of that area. This means fingering her in about a 45 degree angle so to hit the g-spot.
I've also seen instruction videos where one has the fingers all the way inside her, and moving them up and down (instead of in and out). Makes sense, as the area you want to hit is on the "roof" of her vagina (when she's on her back). I haven't had the opportunity to try this technique yet, but I will hopefully get to it soon.
With the first technique I've made girls come, squirt and even cry from pleasure.
You can also massage or lick her clitoris while doing this.
If done properly, vigorously, and for a long enough time, this should make her squirt.
You probably have to fingerfuck her more roughly than what seems natural for you, but as long as you warm her up and make her nice and wet, this isn't a problem. Girls are sturdier than you give them credit for.
