I take it from your question that you are still a virgin so you can't insert your finger very far into your vagina.
You need to find your clitoris which I have nick named a clitty. It is at the top of your vagina. Start by stroking it rythmically in a circular motion softly. Increase the pressure and speed as you get aroused. Continue pointing your toes, clinching your butt cheeks and flexing your legs. It will begin to feel better and better. Keep doing it until you climax or what is called having an orgasm. This will feel like an explosion and shivers of pleasure. You can relax for a minute or two and repeat it.
There is another way to do it and that is with a vibrator on your clitty. Another way that you can do it is to place two fingers on either side of your clitty. It is best to get a hand mirror and examine yourself. You will see that you have a hood over your clitty. It is kind of like a foreskin on a man's uncircumsized penis. You can maniuplate that hood up and down faster and faster and make yourself
come that way as well. Another way that you can do it is to gently pull the hood back and stroke your clitty directly. It will feel much better.
Another way to finger yourself into having a G spot orgasm is to insert a finger inside of your vagina just inside about 11/2 inches in at the roof of your vagina. You start slipping your finger in and out in a rapid fashion gradually increasing the pressure. You will feel a swelling like a rubber eraser grow inside of you. That is your G spot. You keep stroking in and out faster and faster and putting more pressure on it. It will begin to feel like you need to pee. Don't be afraid, let it go, it won't be pee, you will have a G spot orgasm and it is the most powerful orgasm that you can have. You will likely have vaginal gushing of liquid leaking out of you or even squirting. It will feel wonderful and you can repeat this again a couple of minutes later.
Let me know how it goes. I do this for a living as I am a marriage, relationship and sexual coach.