The search box is in the top right of the screen. Start typing in it and it will begin to suggest pages you might be looking for — you can click on one to go directly to that page. Alternatively, if you type in an page title and hit ↵ Enter, you will also be taken straight to that page.
If you type a word or phrase which isn't an article title and hit ↵ Enter, then it will take you to the full search page, which lists articles where your search term occurs. You can also reach this page by typing something into the search box, then clicking the "containing..." link at the bottom of the list of suggestions. For example you might do this if you wanted to search for occurrences of the word "Fish" across Wikipedia, rather than be taken straight to the article Fish.
The search page also allows you to search within the different namespaces. You can choose a preset option (Content pages, Multimedia, Everything) or use the Advanced option to specify the namespaces you wish to search.