How to smooth one part of a mesh? I am following an online tutorial by Blender Guru. I added in a new object (encircled in red below), which I originally applied smooth shading to

However, when I made this part of the overall mesh using CTRL + J, the  smooth shading disappeared. How do I make this particular part circled in red smooth shading, without affecting the rest of my mesh?



EDIT: Thank you I've done that, it's smoothed it a little however you can still see the lines, see below. Is there anyway to make it even smoother?
Select the faces ( if it's a loose part ctrl+L ) > spacebar > shade smooth

edit:
In the latest blender defaults F3  is the search feature instead of spacebar .

Alternatively select an 
object > w > shade smooth