there will be days assigned for locker registration (one day for each building i think).
i don't remember the details but you need to get a locker registration form, pay 40 pesos and line up to choose which locker you want.
it won't be for a while though so try to get acquainted with the buildings and where your classes will be held first before you decide where to get a locker.
ang locker size depende sa building. the lockers in the old buildings [kostka, berchmans, bellarmine (?)] are quite large and can fit a fully loaded sports bag and some more. the ones on the newer buildings (Sec A, B, C) are a lot smaller but enough for books.
Locker forms are available at DSWS, located at ground floor, MVP building. There is a certain period for paying P40 for locker rental. Then, after that, you can register/choose a locker from the building you want on the day of its registration.
example, those who want lockers from building A will get lockers on June X. Those who want from building B will get lockers on June Y. The last day is usually free-for-all locker registration, meaning, you can register for any locker in any building
Usually, lines are long for Berchman lockers. Registration starts at around 9, and ends around 4:30. But lines can start as early 8. For the venue, last first sem, it was MVP roofdeck. then last second sem, it was colayco pavillion (in mvp also)