I'm having a problem on designing a 4-bit ALU that should take two Twos-complement numbers as input(A and B) with a 2 bit selector.

It should perform like this:

OUTPUT selector
add A and B 00
subtract A and B 01
negate A 10
AND (A anded to B) 11

and aside the 4-bit output, there is also 2 1-bit output. Overflow, which outputs 1 if the result of an addition exceeds the output's capability. Underflow, which outputs 1 if the result of a subtraction is smaller than the output can handle.

I'm still having a problem understanding VHDL.

 

You might try working up to it. See if you can get the operation of something more fundamental to operate, like an AND gate. Work up to a 2 bit adder, and then go for the full device.

 

hahahah, thats dead funny, I have just designed one.

It can do:
Add
Subtract
Ex-Or
And
Or
Not
Compare (greater, less, equal to)

I cant give you the schematics just yet but in the future (about a week) I can.