Apple - I started typing in finder the wrong letter now I can't navigate to a file. How to start from beginning?

As nohillside said, you can just wait a couple seconds for the buffer to reset, but you can also hit escape to clear it.

I usually just wait a second or two, and then type the character(s) I wanted to initially.

Press the arrow up or arrow down key to change the currently high-lighted file. Then you can start typing from the beginning again!