help (change 3 chain ring to 2? plus chain device

hi guys ..... slightly new at this ...
i have a scott ransom 20 it has 3 chain rings at the front and i wanna make it into 2, and put a e13 dsr chain device on!
got told you have to shorten the chain and sumhow block of the gear so u cant change it ?

can someone plz run me thru how i do it all or put me on a site? ...

Thanks Guys & Girlys