Where does Elizabeth want John to go, and what does she want him to do there ?

You have not said what Act you are referring to, but in Act II... there is a scene which places Elizabeth and Proctor and Mary Warren talking about what has gone on in the court today. We find that Elizabeth's name has come up, although Mary Warren claims to have helped not see that accusation come to fruition.

Elizabeth wants John to go to Abigail to make sure they talk this out and that Abigail will not accuse Elizabeth.

She says this because

There is a promise made in any bed.

Knowing that John has slept with Abigail, he should have a power over her to make her act accordingly. Elizabeth fears that Abigail will have her killed.