User Activity

Hello ,I am friendly with my neighbour, recently he used my computer to log onto ebayas he had lost his computer password and was locked out.Now, when I go to buy something on ebay, the purchases go to him. How may I fix this?
Newbie, just learning. I am in Perth west aust all auctions on my screen are in AEST Can I, and if so, how do I adjust WA time allowing for daylight savings etc? Thanks for any sage advice.