Every year when the championship ends in July, he starts preparing for the following year.
Literal
Every-year July [in-に] championship [subject-が] end-[when-と], he [topic-は] following-year's preparation [object-を] begins.
選手権大会 ('championship tournament') is a compound — 選手 ('player, athlete') + 権 ('right, title') + 大会 ('grand meeting'). と marks the temporal 'when X happens, Y,' expressing a habitual or automatic sequence. 翌年 ('the following year') contrasts with 翌日 (next day) and 翌月 (next month).