This page explains very briefly how posts work.
Posts are actually the pictures uploaded to the site. You might see users refer to "posts" in some other context, but strictly speaking posts are the pictures.
It's ok to call posts "pictures" or "images", what matters is understanding each other.
Searching a post
There's a textbox labeled "Search" on the side when viewing posts, so searching is a pretty straightforward action.
The workings of that textbox are explained in the help:cheatsheet page.
When viewing a post or the main page, on the side (usually under the search box), there is a listing of tags.
Those tags are usually preceded by a ? (question mark.)
Clicking on it, you will be redirected to the wiki page for that tag.
Not every tag has a wiki page, but the most common ones do.
That page usually explains when it's appropriate to use it (especially for yukkuri-only terms which are never used in other booru sites.)
After the tag name usually there is a number, but due to some issues with the site, it's currently meaningless.
Some users have a "Mode" drop-down menu below the search box.
The most notable options are "Edit" and "Tag script".
The first lets a user edit a post without leaving the main page, while the second allows a user to tag multiple posts without having to navigate to the specific post each time.
For example, using the "marisa -reimu" tag script, each selected post will be tagged with "marisa", and have any "reimu" tag removed.
Some posts have borders of different colors:
The post is waiting to be approved. Usually this means nothing but more work on the admins that have to approve it, but new users can't upload without putting the post in the waiting list.
Anyway, as long as the uploaded content follows the rules, the blue border is just a formality.
This post is a parent of another post. When viewing it, a bar showing the child(ren) post(s) is displayed.
This post is a children of another post. When viewing it, a bar showing the parent post is displayed.