re: Poker & Win in Beginning and Lose toward end
Without more information it is hard to be specific. However, the main difference is that you play with smaller stacks(or short stacks) in the later stages of a tournament.
For short stacks(<10BB), you don't limp or raise 2-3BB. You look for a hand to go all-in. Ideally, you will open-raise so there is the least resistance possible. You would be happy just to get the blinds.
For small stacks (say 10-20BB), play high cards and pairs in position. Don't draw since you don't have the odds
to draw to str8s, flushes or sets. Occasionally steal some blinds since you will not get enough good hands to keep you alive.
With stacks >20BB you should steal more, but don't be a maniac. You can now add SC and small pairs as drawing hands if you are against a stack at least 10 times your pre-flop call/bet. But with SCs you should be in position.
And finally, don't look how you compare to the average. Just compare where you stand with respect to the blinds and the players on your table. There is nothing you can do about the average chip stack. Don't try to "catch-up". If you force the action, you will likely get in a bad situation. At this stage all it takes is one error and you are crippled or out.
Read Harrington's books, play more tourneys and you will definitly improve.