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well im am playing a war wizard with an ac of 19 and im freakng out cause i dont understand what happens when you get an AC about 20. im still not sure im playing him right aswell cause creature have to roll a 19 or 20 to hit me...is that right? thats just how my dm WAS playing..i havnt played with him for a while and when i first did he had only had about 2 games of expirience
 

What edition are you playing? Some weird reverse form of 2nd, or a bizarro 4E where nobody gets bonuses to their attack roll? 

   Nothing happens when you get an AC above 20. (What level are you? It's not particularly hard to get an AC above 20 - for a defender, it's possible by 2nd or 3rd level.) Monsters use the same attack rolls as characters - they roll D20 and add their attack bonuses, so it's not at all hard for a monster with a good attack bonus to hit an AC of 20 or higher. A monster with an attack bonus of +6 will hit an AC of 20 on a roll of 14 or higher.
 Monsters' to-hit bonuses scale by level roughly the same as a character's AC, so a 1st level monster will hit a 1st level character about as often as a 10th level monster will a 10th level character.

By 2nd or 3rd level?  You can easily get 20 a lot of different ways at level 1.  And 21 another couple ways.  And if you are talking a single encounter, 23+ isn't out of the question.  Of course, these characters might not be well rounded, but still.  Without magic items.

Yes, having an AC or other defense score or any DC over 20 is common in 4e.

And yes, a monster would have to "Roll" a 20 to hit you.  But that "Roll" includes what comes up on your d20 PLUS other bonuses (and sometimes penalties).

When you add attack bonuses of all sorts to your d20 roll it's very typical to score over a 20 and hit.

At the highest levels of the game it's not uncommon to have defense scores hovering around 40!

ok thanks guys....and im playing 4e :/        and so i just add up all the attack bonuses and i should be fine?
 

What you really should do is read Player Handbook 1.

Page 11 would seem good, as well as pages 265 - 280.