Completing the Pokédex means 1025 species — alternate forms such as Mega or Gmax do not count separately. Mark the ones you have caught with the ✓ on the tiles in the Pokédex or with the button on a Pokémon's own entry; here you can see how far you have come and which generation is lagging behind.
This list stays in your browser and goes nowhere else. There is no need to log in and no one but you can see it — but nothing safeguards it either: switch browsers or clear site data and it is gone. That is why there is a save-to-file option below, and the button next to it builds a result card to share — your friend sees the numbers, not your list.
This browser cannot save anything (private mode or site data turned off), so the tracker is unavailable. The check marks will not appear in the Pokédex either.
Total
0 of 1025 (0%)
By generation
Backup
Saving to a file is the only copy that gets made — it is worth doing before your list grows. Loading a file adds its check marks to the ones you already have and deletes nothing, so progress from your phone and your computer can be merged into one.