Every player starts with 20 life. Commander starts at 40, Brawl at 25 with two players and 30 with more, and a Two-Headed Giant team shares 30.
The rule
20 is the default starting life total in every format the rules do not list as different (CR 103.4). That covers Standard, the other constructed formats and Limited: a draft or sealed game starts at 20 too, 40 belongs to Commander only.
Commander: each player starts at 40 life (CR 103.4c).
Brawl: 25 life in a two-player game, 30 in a multiplayer game (CR 903.12f, CR 103.4d).
Two-Headed Giant: each team shares one life total that starts at 30 (CR 810.4, CR 103.4a). Teams larger than two get 15 more life for each player beyond the second (CR 810.11).
Archenemy: the archenemy starts at 40, every other player at 20, and life is not shared (CR 904.5, CR 103.4e).
In Life Counter
Pick the format at setup and the app sets the starting life for you: 20 for Standard, 40 for Commander, 25 or 30 for Brawl by the number of players, 30 per team for Two-Headed Giant.
House rules are one step away: change the starting life for the whole table, or for a single seat, for a handicap or a newer player.