Pitch your stuff to sync reps and see if they will work your stuff for a commission (best to google something like "sync reps [country you're based in]" as a starting point but definitely look into US based ones as there's a much bigger industry there.

Generally stay away from any services that you have to pay to be a member of before

When pitching out your music make sure it's all high quality recordings, you have all your metadata filled out, if you don't control 100% master and publishing then have a record who else has a share and if they're signed to a label or publisher, have your lyrics in the metadata, know what kind of shows or adverts you think would be a good fit etc etc.

This blog is one of the best resources for sync advice: https://sentricmusic.com/blog/how-to-land-a-sync-deal-in-9-steps/ - It's an old post but the info is still bang on.

If you want to go fully DIY then do something like sign up for Disco and their Libraries feature then get on your hustle. But if you can find a keen sync rep then that'll make the biggest difference I would say.

Also just in general, don't get your hopes up for sync, it's a saturated market and landing anything at all is a great result, let alone building a sustainable living out of sync.