You need to register and create an account on the official ticket agent of the event in question (Ticketmaster, Lippu.fi, Tiketti, etc.) in order to buy tickets. You will be asked to log in after you have selected your tickets.
If you have problems logging in to the online store, please contact the ticket agent's customer service directly.
Please note: My Live Nation logins are not valid for ticket purchase, but only entitle access to the purchase page during a Live Nation presale.