First you need to understand the Concatenation technique shown.

A1 = Cat
B1 = 123

=A1 & B1 results in
Cat123

Now, we can evaluate each cell individually too as we concatenate values. In your example, it sounds like you want the FIRST LETTER of the value chosen in A1, and the same for B1.

=LEFT(A1,1) & LEFT(B1,1) results in
C1

You can continue to build your concatenation formula the same way.