Hello guys im new here so pardon me if i made some mistakes in making 
this post.

I'm doing my assignment and I need to find left and right nibble of 
unsigned signal.

signal wdata:unsigned (7 down to 0);
This is the signal from i need to take left and right nibble.So how 
should i do it should i declare two unsigned signals with 4 bits and 
trie to slice the wdata or something else.Thank you in advance.

wdata(7 downto 4) is the left nibble. you can assign it to another 
signal or handle it however you want.