Welcome to the community!
ComicWalker can be a good place to check for manga, but the selection can be a bit limited. On the upside: it’s free and legal.
BookWalker is from the same parent company (KADOKAWA) and it’s where I get most of my manga (both English and Japanese). Not free, but the selection is a lot broader.
If both of those fail you, you can check Amazon JP but personally I’ve never had any luck there due to licensing nonsense.
For manga suggestions, check out the Master List of Book Clubs - anything under the absolute beginner and maybe beginner categories would be good to start with. The ones I most often see recommended for beginners are よつばと！, しろくまカフェ and フライングウィッチ, but there’s plenty to choose from - just pick something you’d actually like reading, it’s going to be rough going, so you might as well enjoy the story while you’re at it.
The respective book clubs will also usually list suggestions on where to get the books, so you can check those as well 