need some help with the queen's pawn game

Ok, here's the deal. I am an average player( 1244 highest 1271) and I've got here almost exclusively using 1.e4 open type games. I feel that I have reached a place where I am tactically sound. Whenever I play as black and 1.d4 is played i start sh----ng myself, because I don't know my way around these more slow, positional type games. So, I feel it is time to branch out but would like to know more about the ideas behind this type of play and, more specifically, a system which is easy for a beginner in this type of game to practice. Which 1.d4. opening should I start with and what should I keep in mind during this?

Maybe play modern/pirc/kings indian defence against 1. d4 as black instead of learning a whole set of 1. d4 openings?

Play a Semi-Slav.  1...d5  2...e6  3...c6

This is often referred to as the triangle formation.  White really can't stop you from getting here.  White can however get into the Marshall Gambit from your move formation and you better be prepared.  The gambit is sound and dangerous for Black.

I would like to mention that although having a repertoire that you are comfortable with is important, you need to study tactics more.  I am rated about 1500 now and spend the bulk of my time studying tactics.  I know folks rated significantly higher than me that spend most of their time with tactics as well.

Um...it depends normally Nimzo Indians, Benkos and Benonis are common moves. I would not usually play d5, i think that will lead into a Queen's Gambit which is goot for white. A typical Nimzo Indian perhaps? (This following sequence has more than 1 opening so make sure you see the alternate variation)