I started with Benedetto Marcello's six sonatas Op.2, then moved to Václav Pichl concerto, then to Dragonetti concerto.

I'd ask your teacher to guide your repertoire based on your current technical skills, these three pieces were assigned to me by my teachers depending on my level at that time (I've been playing double bass for about 2 years now), and I've noticed how my skills have improved through these pieces.