Art supplies can be very personal, and everyone will tell you something different, but if you don't know where to start, here are a few of my absolute favorites for creating.
Gesso is a must-have for painting and art journaling, the brand I use typically is Liquitex. I buy it in larger containers and add smaller amounts to a jar for ease of access, plus it keeps it cleaner. I think most brands would be fine I just know and love Liquitex.
I typically use Golden Matte Medium as my all-things collage adhesive, any time you need to stick something down this is perfect. Liquitex also has a great Matte Medium that I occasionally get. Matte is the finish, it also comes in gloss, but that to me is more for a final coat.
Gel Matte Medium is great for sticking things that are a little thicker or are not completely flat. It is also great to add to your paint to make it a bit more heavy bodied so you can add some texture. Again, I use Golden, but I am sure...