Hi there! You seem to be thinking in the right direction! When writing for comics it is important to keep in mind the visual aspect, and what can be achieved through drawing. I produce comics on my own, writing AND drawing, so the process I employ is very different and much more drawing based than that of a writer/artist duo, but I used to write scripts in school for a variety of projects. Depending on who you end up working with, be it a friend, a stranger, or even if you draw it yourself, your process will change depending on how you and/or your partner work best.
Generally, I would probably leave character descriptions out of the script. Aside from any incidental changes(small visible changes ie. injuries, outfit changes, props) you can talk with your artist partner before hand about drumming up some designs that you both like. Background descriptions would be super helpful at the beginning of each scene.
Your descriptions of things don't need to be as prosaic as they would be in a novel. The main goal is to have them be clear, so that the artist will have a good image in their mind of what the scene requires. Script writing is less a book, and more a list of things that make a story. Its not bad to get a little flowery if you are trying to set a mood, or explain how a character is acting, so dont think you can't get fancy if you dont want to, but sometimes it's not necessary. Your artist partner will likely have some good ideas in the way of acting and layout. Write as beautifully as you want for dialogue though because that is where your words will really shine for the reader. The description is just to communicate with the audience that is your partner.
This is an image of some pro scripts from Mark Waid and Chris Samnee's run of Daredevil as an example of a comic script(with some drawings by the artist):
You can get as detailed as you like with your scripting, down to calling the number of panels on the page, but this is not necessary, and may be difficult for you to visualize for your first try. I am more inclined towards drawing than writing, and even I took a few years before I was able to accurately block out pages in my head. The most important parts of the script should be the flow of the action, conveying scene mood, and the character dialogue. Although, I will say that especially when working with someone who is not used to your style yet, more detail could not hurt.
Since this would be your first stab at working with an artist on a comic, you may want to work together to figure out the page and panel breakdowns. You should feel free to make suggestions to how things will look, but remember this is a partnership, and if your artist is so inclined, they may want to bring a bit of their own flare to the visuals.
I think you should expect some back and forth at the layouts stage. This is a big part of any team-based creative endeavor. If you and your artist partner are receptive to eachother's ideas and trust eachother and are respectful, you should be able to do fine work! Communication is key!
There are a lot of articles about how to write scripts for comics out there on the internet that you can refer to for formatting, and if you are looking for script writing software I can recommend Celtex, which is a free program (available on mac or web-browser) that does have a half decent function for writing for comics.
Poke around and do some experimenting to see what comics feel like before starting out in earnest! Best of luck and happy writing!