To create a believable setting takes attention to detail and patience. You might not be able to create the perfect setting for your book right away. You might have to fine tune it over the entire period of writing your book.
It normally helps to write about a place you are familiar with whether it is where you live or an area you have visited. This way you already know some of the things about the area and if you need to research it won't be for the whole area. I have read books by top authors and when the book is situated in the town I live it, you just want to laugh at some of the things they believe about the area. I lived in Las Vegas for 11 years and many people wrote books with the setting in Vegas. Either they didn't visit the city and just wrote about what they thought about the town or if they did visit they never left the strip. One of the books I read talked about leaving the strip and traveling to this one area of the town and their directions that were listed in the book took them to a completely different area of town and had two roads cross that ran parallel. These are the kinds of things you need to look at when you are writing a book. If you don't know the area don't just assume things about it, or use information you received from TV and the movies. Normally it isn't completely accurate.
When you are creating a setting that doesn't exist then be mindful of what information you put into the setting in prior areas of the book. I have found if I write down what was in an area it helped me to remember when I had the characters walk through the area in a later chapter.
As with all writing creating a setting is all about the details. You need to make sure it is consistent throughout the book not just the part you are writing. When you are able to do this then you will be able to add details to the setting to make it come to life. Which will make it real for your readers and this is what you want to do to keep them coming back for your future books.