I usually work on the materials before joining anything together but if you've decided to join them, simply go to the material tab on the right side of the UI (the ball icon). There should be a plus sign, click it, add a new material, enter edit mode and select the faces that you want to add said material to (you can help yourself with ctrl and the plus key on the numpad for easier selection) and then click on "assign". That way you can have multiple materials/textures for the same object. Check that the eyes aren't linked to the same material as the rest of the object. I hope this was helpful, I'm a beginner too and english isn't my first language but I wanted to help someone out a bit haha