You can translate the website yourself, or at least I can right click, select "translate to English" and it sort of works. The website is a general site for several attraction by the Patrimonio Cultural.
Digging around, there is a tab for an online ticket office, but it appears to be under construction, or at least not active.
Your best option is to purchase a combo ticket that includes the Archeological Museum, Monastery, and Tower. The Rick Steves guide suggests buying the ticket at the Archeological Museum where the lines are shorter, and avoid the ticket line at the Monastery.
With that, simply buy when you arrive to see the sights, there are no timed entries, buying at one sight may let you skip the ticket line at the others, but cannot guarantee that.