Hi there, Iris!
Thank you for your question.
If you don't have a real estate agent assisting you, that is absolutely the best bet for you at this point - find an expert to explain how the whole process goes and to assist you throughout the home search.
Generally speaking, you may request a showing at any time.
In this market, because of the complexities of financing, I would strongly urge you to contact a few lenders before you start looking in order to nail down your price range.
Further, any agent who IS listing and/or showing a property is going to ask you whether you are pre-approved yet - it's an important question.
It works out way better for everyone if your answer can be "Yes!"
Best of luck to you, Iris!