Get context

$ kubectl config current-context
my-context

$ kubectl config get-contexts
CURRENT   NAME               CLUSTER            AUTHINFO                             NAMESPACE
...

Set context

$ kubectl config set-context my-context

Namespace

You need to run commands in a namespace.

e.g.

$ kubectl get pods --namespace=my-namespace

Or set it globally for the context.

$ kubectl config set-context --current --namespace=my-namespace