You can find various search modifiers in the help center. Or simply by clicking on "Advanced Search Tips" after you search for something.
Specifically, to find posts by a specific user you can user
user:userid. And to find your own posts you can use
Notice that if you go to some user's profile page, the search query
user:userid is already pre-filled in the search bar.
For example, you can search for user:27004 or user:me. (The first one searches among the posts of that particular user - I chose the OP of this meta question as an example. The latter automatically populates the search box with your own user id and searches for your own posts.)
Of course, you can combine this with other keywords, tags, search modifiers (such as
is:a for answers,
is:q for questions), etc. For example, user:me is:a [ct.category-theory] returns your own answers in the tag ct.category-theory. Similarly, user:me is:q url:"*arxiv.org*" returns my questions containing a link to arXiv. Or user:me quantum searches for the word "quantum" in your own post.