How to make her know that I need sex?

Kiss her neck and nibble on her ear. That should get her juices flowing.

My opinion? Just jump her dude!

Well I suppose if "her" is your girlfriend, you just try to keep it at physics at first (try to start it by kissing, cuddling and etc) and if she is not responding the way you expect, why not to tell her? But be aware that by telling, you are taking a risk of getting a "no" as well. A very little percent of risk though! And if "her" is just some girl it's better not to tell directly but to tempt her by the ways you guys know better :D anyway, "I want to have sex with you" is better be told in more seducing sentences, and is better not to be your first choice.

Try telling her physically first, by doing innocent touching, leading up to more intimate touching.
If she still doesn't get it, it's okay to let her know with words. But say it with some class. Don't just say "I wanna have sex".

Uhm, your a guy. Right? If that is the case then I can tell you without a doubt that /every/ woman on this planet, and I am sure other planets as well, knows that you want or need sex.

Having said that, a woman will know almost instantly if you will ever be allowed to enter the secret garden so now your task is to knock on the door.

Next time you are making out, take it a bit further than in the past, see if she responds in kind. Again, take it a bit further, see if she responds to it, a bit further again, she responds and if she feels the same about you, you'll both be naked on the floor going at it like two bunnies on easter sunday. :)

Believe me, women don't kid themselves into thinking that the guy is just there only for the witty conversation and good company. Heck, they are just as sexual - if not more so than us - but they need to exercise caution to weed out the losers, chimps and creeps and many times it is up to you to initiate the sex. /You/ need to initiate it in order to find out.

. You could also tell her that you want to get laid. Depending on the girl, you may get lucky but I think being this direct, unless you have been in a relationship for a while and sexually active already, she may be put off by this.