You dont have to be alone to do it. just go to the bathroom close the door and stand/crouch over the toilet let your cum shoot into the toilet. You orgasm at the same time u cum. To get a erection just think of girls/sex etc. When u get the boner/erection keep thinking of girls etc. then start rubbin ur dick(put ur hand around ur dick then rub up and down quickly...DONT STOP!). U will feel the cum coming up ur dick after a while(almost like when u piss, but not really the same)
Just do it man!! Dont worry about being alone. Just do it somewhere private.
When u have finaly done it you will understand all of this!!