Jersey's of questionable taste can be had for next to nothing on the bay, a decent base layer is an advantage, and as my legs are not for public consumption, I tend to ride in 3/4 bib knickers. It is also worth getting some road shoes, as they are stiffer and narrower than mtb shoes. I have a very pretty pair of Fizik R3's which I got given as they did not fit my son, or the bloke he bought them from for £80.
You can really infuriate (some) serious roadies by riding in your mtb helmet, complete with peak. Gloves, glasses etc are all pretty common stuff.
One bit of safety advice: Get the brightest, most dazzling flashing rear LED light and use it every single time you ride. Some car drivers are completely oblivious to cyclists, and a nice bright flashing light does tend to get their attention, especially if you angle it correctly
I am really enjoying my road riding. I try to get out every other evening, and am working myself progressively harder. I also have.... ermmm, a Garmin
It is really useful for recording ride distances and times, but if you have a fancy phone, you can download something called and ape, or is that an app, that does the same thing.
The first thing you will notice is how far and how fast you can ride, in comparison to an mtb.
Welcome to a whole new world of bike riding.
"Try not to punch any clowns"