Bold text
To make a block of text (or just a single word) bold you can use the B button in the toolbar at the top of the text input area. The simplest way to do this is to enter the text first, then highlight it, and then tap/click on the B button. The result in the text input area will look like this:

Italic and underline work in just the same way.
URLs (web addresses)
Web addresses begin https:// and are what you type into your browser to go to a webpage. If you type an address beginning https:// the software will automatically render this as a clickable link like this https://insect-project.org. If you want to make a word or words appear as a link to a webpage (for example The InSECT Project), the BBCode to do this looks like this:

Lists
Using BBCode you can have bulleted lists and ordered lists. Ordered lists can be numbered using Arabic or Roman numerals, or lettered (both uppercase and lowercase are possible - use 1, I, i, A or a after the = sign as you wish):
- Item 1
- Item 2
- Item 3
- Item 1
- Item 2
- Item 3
- Item 1
- Item 2
- Item 3

Nested lists are possible:
- Item 1
- Item 1a
- Item 1b
- Item 1c
- Item 2
- Item 3
The simplest way to include an image with your post is to upload it as an attachment to your post using the Attachments tab below the text input area.
If you want to embed an image in your post, the code is inserted using the insert image button in the toolbar - but note that the image must be hosted online and you must know the url for it.
The BBCode looks like this:

And here's the result:

You can preview what your post will look like before you submit it using the Preview button at the bottom of the text input area.
There's a comprehensive guide to using BB code to format posts here. You can access this at any time when you're making a new post by going to the FAQ linked from the list underneath the emojis on the right hand side of the text input area.