Go to the train station at the gates and head towards the RER lines, they're clearly labelled (RER A) and buy a ticket for Val D'Europe. You can get them from the machines (change language to English>"Tickets for Paris Region from this station">V>"Val D'Europe") or from the ticket booth. Tickets cost €1.45 for adults.
Go through the gates putting your ticket in the machine, make sure you keep your ticket as you will need it to get out of the station at Val D'Europe.
Board any train heading to Paris (they all do!) and get off at the first stop (it's about 2 mins away) When you get to the Val D'Europe station you will notice there are 2 exits make sure you take the one labelled shopping centre (it's very clear) When you exit the station turn right and the shops will be in front of you.
Hope this helps!