B="Good"&"Morning" would work. All you have to do is put quotation marks and an & between each word you wish to join. If you were joining two specific cells you'd typically use =A1&B1 (with the A1 and B1 being your cell names) though for no space and =A1&" "&B1 to include a space.
You could also use the cocatanate method to join specific cells together like so: =CONCATENATE(A1, B1) or =CONCATENATE(A1, " ", B1) to include a space.
