What am I always allowed to do for the player?

A caddie may always do these things, when the Rules allow them:

• Carry, transport and handle the player's clubs and other equipment, including driving a cart or pulling a trolley.
• Search for the player's ball.
• Give information, advice and other help before the stroke (within the limits on lining up and standing behind the ball).
• Smooth bunkers and take other actions to care for the course.
• Remove sand and loose soil and repair damage on the putting green.
• Remove or attend the flagstick.
• Mark, lift and replace the player's ball on the putting green.
• Lift the player's ball when it's clear the player is going to take relief.
• Clean the player's ball (when the Rules allow it to be cleaned).
• Remove loose impediments (leaves, twigs, stones) and movable obstructions (rakes, bottles).

Source: Rules of Golf (2023), Rule 10.3b(1).