Little bits of knowledge I have gained through experiencing 21 years of life-
Have faith!  In God, in yourself, in your dreams....etc.  Never settle for less, and do what you think is right for your own life. 
Smile a lot!  -To friends, acquaintances, random people you pass in public....
It's amazing what a little smile can do.  Almost always they will smile back, and if not, it at least probably made their day a little brighter.  It helps people feel more comfortable around you, and it's just enjoyable.  Smiling not only makes the smilee feel better, but it helps you (the smiler) also!  :-)
Find something you like and stick with it.  Now don't get me wrong, it's great to try new things, and I think everyone should.  But it's wonderful to keep your favorite things around.  They make you feel comfortable and they just give you a good sense of stability.  So let people know you're favorite things and don't ever give them up.  To see a list of my favorite things click here.
Some of my favorite verses:

"Wait for the Lord;  Be strong, and let your heart take courage;  Yes, wait for the Lord."                 - Psalm 27:14

"Make every effort to live in peace with all men and to be holy; without holiness no one will see the Lord."
                                                                         - Hebrews 12:14

"If possible, so far as it depends on you, be at peace with all men."                                                               - Romans 12:18

"Then I heard the voice of the Lord, saying, 'Whom shall I send, and who will go for Us?'  Then I said, 'Here am I.  Send me!'"                                                               - Isaiah 6:8 
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