So I have two users. First is user C and second is user S (customer,seller). Anyone know how can I display, if user is category C than the label "YOU ARE CATEGORY C" will appear and if the user is user S the label "YOU ARE CATEGORY S" will appear. I use firebase in storing the user category
What if a user is both a customer and a seller?
Nope it can only be either Customer or seller because I set it that way.
Its just a simple school project 
You should use If..then statements. A nice tutorial is in Programming Your App to Make Decisions that explains the concept.
Something like:
If CATAGORY C then display Label.Text = You are category c else Label.Text = You are category S .
Create some Blocks. If you run into trouble, post them here and someone will provide specific advice. Exactly what you need to do depends on your log in code so far.
Thank you so much