The first step in choosing the best site is narrowing your choice down to a handful of possible sites.
Go to the all sites page and use the list view. Then narrow your options down by selecting a category up the top. Read through each of the site titles and descriptions, and write down the sites that sound appropriate.
Go to each of the sites you wrote down, and read their tour (sitename.stackexchange.com/tour). This will give you an idea about what sort of questions are on-topic for that site. Also consider searching the site's tags and reading their tag wikis (sitename.stackexchange.com/tags); they will also give you an idea of what sort of questions you can ask.
By now, hopefully you will have figured out which site you can ask your question on. If you're not entirely sure, try dropping into the site's main chatroom, or asking on their per-site meta.
If you are debating between two sites, or you haven't found the site you are looking for, ask a question here on Meta.SE. That's one of the reasons we're here. Remember to tag your question support and site-recommendation, and show us the question you want to ask.