To create a Couchbase client instance, the URI of any available node in the cluster is needed, as is a bucket name and password (or cluster credentials).
couchbase = Couchbase("http://192.168.56.2:8091/pools/default", "default", "");
The default Couchbase Server installation creates a bucket named "default" without a password, therefore the third argument is optional when used with this bucket. If you created an authenticated bucket, you should specify those values in place of the default settings above.
The construction of a Couchbase client instance does require some overhead as the cluster topology is discovered, so as a best practice the client should not be created more than once per bucket, per application.