The first ninety days set expectations that are hard to change later. Treat them as a deliberate project.
Weeks one to two: learn, do not fix. Meet everyone you will work with, ask each of them what would make their job easier, and write down every process you touch. Take notes publicly so people see you paying attention.
Weeks three to six: deliver something small and visible. Not a reorganization. A report someone needed, a bug that annoyed the team, a document that did not exist. Credibility comes from finished things.
Weeks seven to twelve: propose one real improvement, backed by what you observed and who you talked to. Now you have the standing to be listened to.
Throughout, over-communicate in writing. A short Friday note listing what you did, what is next, and what you are blocked on takes ten minutes and prevents most misunderstandings.
Ask your manager directly, in week two and again in week ten: what does a strong first ninety days look like to you? Then go do that.