you take the knight. if you pin a knight to a king in the opening you generally should be prepared to trade for it otherwise you're wasting your time because the king quickly moves out of the pin when it castles.
Nimzo-Indian defence question (noob)

what stops white from simply advancing the pawn on a2 to a3 attacking my bishop,
Nimzowitsch called this "Putting the question" to the bishop. The idea being that the bishop pins and threatens the knight, but will the bishop take the knight if the question "a3" is put to the bishop ? The short answer is yes....
The longer answer is that strategy involving double pawn complexes outlined in Nimzos "My System" are useful to read a little later for you. I suggest you read Chernev's "Logical Chess" and his "The Most Instructive Games Ever Played" before "My System".
The Nimzo Indian was a defence worked out later in chess evolution than the more symmetrical responses 1d4 d5. It's often been said that beginners or nearly new beginners might consider mirroring this chess evolution in their own progression. Perhaps starting out with 1e4 e5 or 1d4 d5 with both colours... but long term the Nimzo Indian is a great defence...
hey guys i am a complete noob at chess and the people i will be verusing will be bad also i would say around a rating of about 1100 max?
i am learning some openings for black as its my weaker side and i think the nimzo-indian defence is a good counter to the queens gambit. if this play happens
1. d4 Nf6 2. c4 e6 3. Nc3 Bb4
what stops white from simply advancing the pawn on a2 to a3 attacking my bishop, if i back the bishop to a5 then white could simply move the pawn on b2 to b4 threating my bishop again, if i let this to happen it feels like i have no control on the match.
to summarise i am wondering if there is a way a can make my oppenent pay if he goes for my bishop.
thanks.