Question: I don't really have a high self...

It sounds so simple but strive to be positive about everything. Don't allow negative thoughts to invade your mind. Talk to people. Just be friendly. Smile. Confidence is key. Aim to be a confident, happy person and you will attract other happy people. Relax and have fun!

First, you need to find things that you like about yourself. And put them on a little card to carry around with you. Things like "I am____"

I completely had this problem and it really held me back for way too many years. Honestly I sought out a therapist and simply talked through it all and then I was just over it. Can't say it happened quickly but it did happen and I started to believe in myself. I started to see myself as others saw me. I also had the opportunity to fix a few personality flaws that could be a bit annoying. Not saying you have any of those - that could just be me. But now I don't feel those feelings about myself. I cannot tell you that I'm Ms. Happy-Go-Lucky but I'm 100 times better. Everyone can tell you a hundred times over that you are amazing but until you believe that about yourself it's not going to matter at all. And there's nothing wrong with seeking out some help. It's awesome getting the opinion of a non-judgemental person who's only got your best interests at heart. You can get a referal from your general doctor too.

find something that you dont like about yourself eg if you dont like your hairstyle or body and change it so you like it
i used to have quite low self esteem because i didnt really like my body but once i started going to the gym my self esteem sky rocketed :D
i also spand alot of time with people who make me laugh and feel good about myself and dont bother with the others even if they were good friends once

if you put yourself out there and the guy you asked out rejects you just think your better without him and he doesn't know what hes missing

I had the same problem but then I met one great girl, we are gr8 friends now. She taught me to value myself, to say I AM great and nice and if somebody doesn't see it, it's their problem. For the beginnig, everytime you see a mirror, try to find something beautiful about you- it doesn't have to be just appearance- and say somethinh like: "Wow, I've got so nice eyes" or:" I am gr8 in making people happy and that makes me special". You prolly won't believe in from the start but believe me, once you'll find out that you really are great and you do believe it. And you won't even notice when you started to believe in it. Just repeat it for yourself and it will become true. Also, if you've got best friend, ask her/him to compliment you on anything nice/good/remarkable that you did. Even if it's just a good mark in school.
Good luck! And you ARE gr8, believe it ;)

I don't want to sound morbid but go to your doctor and have an evaluation. Getting help from a psychologist/psychiatrist is the best thing that I did. Some of the 'warped' thinking that I had going through my mind did kick my self esteem. Getting a trained opinion from someone who wasn't involved in my life helped so much.
Take this as a positive. You have taken a big step and know that you need advice. Things can only get better from now on.

List all of the positive things about yourself. Look in the mirror and say, " I look great today." I have the same problem. Friends that are more daring than you will also help you with self -esteem, it has for me anyway. But there is so much about you for people to like, so show it! Rejection is gonna happen no matter what you do in life, so its okay to get rejected sometimes. Do something you are good at to get your esteem up too. For me, it's acting because I get to be someone else besides me on the stage. Try something like that. And remember, you are great.