We are using Flarum to power our forums which is a fast and light forum solution but might not do everything the way that you are used to in other older web forums...
We do not have 'categories' but we do have 'tags' which work exactly the same way. When you start a discussion you can give it one or two tags, for whichever categories you think it will fit into best.
In the post editor you can do almost everything from the toolbar at the bottom... like insert headings, bold text, lists, quotes, etc. You can also upload images using the 'Attach' button.
If you type a web address it will automatically be turned into a link and if you type '@' followed by a forum username then they will be ''mentioned'.
To add emoji, either click the button or type ':' then start typing to search for the emoji you need. For example ':cheese' = 🧀 & ':green_heart' = 💚. If you are using a phone then you can also just use emoji from the keyboard like you would anywhere else.
You can also use Markdown or BBCode in your posts if you like. You can find a simple guide to markdown here: Markdown Cheat Sheet
Oh, and if you don't have time to finish a post then you can save it as a draft using the disk (anyone remember those) icon at the top of the post editor.
If there is anything else you need to know or think is missing from this little guide then post it below and we'll update and add it.