As a new hire how to deal with legacy codebase and introduce standards

Recently accepted an offer to join a small size company as a mid developer. From the internal scoop and the interview I already know the code base is not great. There is a lot of technical debt, legacy tooling, poor coding practices and a lot of junk. I don't want to give up and just write more cruft or throw tantrums and pester everyone to re-write stuff.

If you are in my position, How would you approach ? What are the methodologies you follow to get yourself comfortable to start contributing to the existing codebase ? How do you gain existing team's confidence so as to persuade them to make some changes instead of sounding bossy ?

I faced similar situation in previous position and find it really difficult to get started. Any pointers are welcome!