Queen's gambit declined, lasker variation, is quite a good starter defense to 1.d4 IMHO. Solid and strategically straightforward (and has been used by world champions).
Whatever you choose, find some games featuring that defense and play over them just so you can see some of the usual ideas.
If you adpot this though, off the top of my head, you'll also want to be aware of the exchange variation of the queen's gambit, and the catalan, although in any case, your setup will generally involved the moves d5 e6 Nf6 Be7 0-0
I am going to participate in my first chess tournament soon. I am trying to learn enough openings to not be caught with my pants down too many times. I have learned the Scandinavian Defense, the English Opening and now I need a response to 1.d4. What is the best or at least easiest to learn?