If you thinking like RDBMS, you can’t create primary key. Default primary key is _id.
But you can create Unique Index. Example is bellow.

db.members.createIndex( { "user_id": 1 }, { unique: true } )

db.members.insert({'user_id':1,'name':'nanhe'})

db.members.insert({'name':'kumar'})

db.members.find();

Output is bellow :

{ “_id” : ObjectId(“577f9cecd71d71fa1fb6f43a”), “user_id” : 1, “name” : “nanhe” }

{ “_id” : ObjectId(“577f9d02d71d71fa1fb6f43b”), “name” : “kumar” }

When you try to insert same user_id mongodb throws a write error.

db.members.insert({'user_id':1,'name':'aarush'})

WriteResult({
“nInserted” : 0,
“writeError” : {
“code” : 11000,
“errmsg” : “E11000 duplicate key error collection: student.members index: user_id_1 dup key: { : 1.0 }”
}
})