One tip that works great in Discourse is to use the Hide Details spoiler to create a longish list of FAQ-style items without presenting your new member with a vast wall of text. Using question format may be more eye-catching. Example:
How to use this forum:
Can I download ferrets from this forum?
No, only goats and hamsters.
Also check our array of fine roosters at the forum store…
What is the forum dress-code at weekends?
Clown shoes and bowler hats please.
Suggest they bookmark the PM, as Discobot does iirc, so they can find it again - put graphics showing how under a spoiler, and title it to also show how opening spoilers works. Example:
Click or tap here to find out how to save this message for later.
Uploaded graphic showing how to Bookmark, and where to find Bookmarks later on
In general, consider using infographics that “show, don’t tell” and which people can also click to take them to the place you want them to be, for example, an introduce-yourself style topic. Arguably, text can sometimes seem authoritarian, a meme-like image that is also a link is sometimes going to go down a bit smoother.
Invite them to jump right in, and consider asking 3 questions suitable to the tone and subject of your forum, request they reply to one or all if they choose. Include a graphic showing how to make a New Topic and make that graphic into a link to the Category you’d like them to reply into.
Keep things about the person reading, not about what the forum (as a monolithic and unrelatable entity) expects from them: this is a concept advertisers use, and I have had success with it in multiple areas of life (not just forum things).
Consider what things your members innocently get wrong most frequently, and also which rules you see broken most often, and if it’s authentic to your style and the nature of your forum, consider addressing those by describing the desired behaviour in a positive way.
"Bowler hats are very important to our members, so please remember to wear yours when posting! " is nicer than “Members who post bare-headed or in flat caps will be bannned on sight!!1!!”
But personality types among people reading this will differ, so mix a little carrot and a little stick. Be judicious in use of your authentic voice, if the message is sent from a human admin account.
Just some “top of my head” ideas, styles and forum cultures vary enormously so these are just things I have found work well, ymmv.