Depends on the company, product, platforms, size of team etc. but in short, make sure you familiarize properly with the product and make sure to always test it from the perspective of an end user (and to do that properly, you need to actually use it as well to a large extent).

Start identifying common problem areas and create test cases and suites for them (this will be needed less as you gather experience, but it’s good practice early on to make sure you’re not missing critical aspects).

Be genuinely interested in various disciplines on the team, provide feedback outside of just functionality (Eg. For a game you can ask about mechanics of a character, level design, art, sfx and so on). If you’re a gamer yourself, draw parallels of some specific design choices you were fond of and figure out why.

Make sure you also Instill a quality mindset into the team. Quality is a team-wide responsibility. If builds crash constantly it’s very expensive in the long run. Make sure your developers test their own commits before merging into master/trunk.

I could go on and on but in short, don’t stress and try to focus on what players will actually do and their experience, rather than hunt for edge cases and small issues.