You start with the appropriate number of XP and gold:
Rulebook p.42 wrote:
A player can start a character at any level equal to or lower than the prosperity level of the city (see Gloomhaven Prosperity on p. 48 for details). If a player starts a character above Level 1, they should go through all the steps outlined on p. 44 for each level increase in sequence, up to and including their chosen starting level. In addition, a newly created character receives an amount of gold equal to 15x(L+1), where L is their starting level. A character starts with an amount of experience equal to the minimum required for their level (the number listed below that level on the character sheet).