What do I read as my first-ever comic? I am an MCU fan who has never read the comics, but I want to.

There are lots of good options! If there are specific heroes/heroines that you like best, starting off with their comics wouldn't be a bad idea. It can be difficult to pick a place to start, because the comics have been in publication for such a long time, but it's advisable to try to start at the beginning of a plot line if possible, though if you feel you could stick with it it's perfectly acceptable to just jump right in at a random point. There is no "right" way to read and enjoy comics; whatever you can comfortably afford and follow along with is okay.