This post is going around again (with an unfortunate syntax error on it - this version is correct), so I thought I'd add some information that folks have mentioned wanting in the notes.
Any time you want help with how to use AO3, look for the question mark bubbles. Clicking or tapping on those bubbles will open up a pop up window with information on that specific part of AO3. For example, here's the information for Include filters.
2. When you log into AO3 for the first time, you'll have a banner on your dashboard that gives you a ton of links and information that's useful for new users. A lot of people will dismiss this banner without ever reading it, but you can get it back at any time.
Go to your Preferences, scroll down to Misc. and check the box next to Turn the new user help banner back on. Update your preferences, and the banner will return.
3. When you first tap or click into the search in the header bar, a little popup tip appears below the text box. This is showing you an example of a search that you can do.
4. If you've never seen the filters before, click or tap on any tag that you're interested in. On a wide screen like a laptop, the filters will appear on the right hand side of the screen. On a narrow screen like a phone, you can tap on the Filters button for the menu to appear.
5. The FAQ has a series of questions related to Searching and Browsing on the Archive. You can find the FAQ in the dropdown under the word About in the header.
6. You can also just click on stuff and see what happens. For example, if you're wondering what the four icons at the top corner of each fic blurb mean, you can just click on them and get a popup with the key. Same thing if you're wondering about the icons at the top right of a bookmark.
7. I almost forgot ☝️If you want to see fics based on the primary pairing but you're okay with additional ships being in the fic (so otp:true won't work for you) - there's a script for that! AO3 Only Show Primary Pairing.
This post is extremely long now, but if you're wondering about other things just drop them in the notes.