You'll probably want to setup your database and add a few users. I will briefly go over how to setup the tables using the schema provided with kamailio.

loadmodule "auth.so"
loadmodule "auth_db.so"
loadmodule "usrloc.so"
loadmodule "registrar.so"

modparam("auth_db|usrloc", "db_url", DB_URL)
modparam("auth_db", "use_domain", 1)
modparam("auth_db", "calculate_ha1", 1)
modparam("usrloc", "db_mode", 3 )
modparam("usrloc", "desc_time_order", 1 )
modparam("usrloc", "nat_bflag", 1 )
modparam("usrloc", "timer_interval", 5 )
modparam("usrloc", "use_domain", 1)
modparam("nathelper|registrar", "received_avp", "$avp(s:rcv)")

# -------------------------  request routing logic -------------------
# main routing logic

route {
        # per request initial checks
        route(SANITY_CHECK);

        # CANCEL processing
        if (is_method("CANCEL")) {
                if (t_check_trans()) {
                        t_relay();
                }
                exit;
        }

        route(CHECK_SOURCE_IP);

        ##################################
        ### HANDLE SEQUENTIAL REQUESTS ###
        route(WITHINDLG);

        ###############################
        ### HANDLE INITIAL REQUESTS ###
        t_check_trans();

        route(REGISTER);

        route(INVITE);

        route(RELAY);
}

route[REGISTER]
{
        if (is_method("REGISTER")) {
                # auth user/pass
                if(!www_authenticate("$fd", "user_extension")) {
                        www_challenge("$fd", "1");
                }

                if (!save("sip_registration")) {
                        sl_reply_error();
                }
                exit;
        }

}

# Handle INVITE + REFER
route[INVITE]
{
        # record routing for dialog forming requests (in case they are routed)
        # remove preloaded route headers
        remove_hf("Route");
        if (is_method("INVITE|REFER")) {
                record_route();
               # auth user/pass
                if (!proxy_authenticate("$fd", "user_extension")) {
                        proxy_challenge("$fd", "1");
                        exit;
                }
        }
}