Who serves next in padel?

Service alternates between the two pairs game by game, and within a pair one player serves the whole game. So if you served the first game, your partner serves the third, and the opposing pair serves the second and fourth. A pair can change which of them serves first at the start of a new set. Inside a tiebreak the pattern changes: one point, then two points each.
Who serves first in a padel match?
Nothing in the scoring decides it. Before the match the two pairs spin a racket or toss a coin. The pair that wins the spin picks one of two things: serve or receive, or which end to start on. The pair that loses it picks whichever of the two is left.
Only after that does the rotation below take over. A pair that chose to receive still serves the second game, and from there the alternation runs to the end of the set.
The serve itself
The serve is underarm. The server bounces the ball behind the service line, strikes it at or below waist height, and the ball has to land in the service box diagonally opposite. Two attempts are allowed, exactly as in tennis.
A serve that lands in and then hits the side or back wall is still good. A serve that hits the wire fence before the ground is a fault. That difference is padel specific and it decides a surprising number of points.
Rotation through a set
Games alternate. Pair A serves game one, pair B serves game two, pair A serves game three. Within pair A, one player serves game one and the other serves game three, so each player serves every fourth game.
At the start of a new set a pair may swap which of them serves first. The pair that serves first in the new set is the pair that did not serve the last game of the previous set.
Ends change after every odd numbered game. The score being on a scoreboard makes this trivial. Kept in four heads, it drifts.

Rotation inside a tiebreak
The tiebreak breaks the game by game pattern. The player whose turn it is serves one point. Then service passes to the other pair, and every server after that serves two consecutive points.
When the tiebreak finishes, the next set starts with the serve going to the pair that did not open the tiebreak. This is the single most argued point in club padel, and it is the reason a scoreboard that tracks serve is worth more than one that only tracks points.

Questions people also ask
Can partners swap who serves first in padel?
Yes, at the start of each set. Within a set the order is fixed, so each player serves every fourth game.
Who serves first after a tiebreak?
The pair that did not serve the first point of the tiebreak serves the first game of the next set.