Can't create user in oracle

i’m using oracle xe edition at ubuntu 14.04. I’m trying to create a user using my sysdba account but when I type the command

create user 'myuser' identified by '******';

it gives the error message “ORA-01109: database not open” then I type:

alter database open;

it gives the error message “ORA-01507: database not mounted” then I type:

startup mount;

it gives the error “ORA-01081: not possible to start ORACLE when it is already running” (something like that)

can someone help me?