Hi, i'm lazy to backup my wallet every time..
So i want to ask..
How to see my private key? 
Because that is easier way to backup my wallet on a different PC if i know the private key..

Most of the times, when you send some coins a new address is generated to hold the change.
So it's not recommended at all to just save a private key, you may lose some coins if your PC breaks. You definitely need to backup the wallet.dat.
Anyway, you can get the private key of an address with the command
dumpprivkey.