You should use an 8x8 array:
DIM BOARD[8,8]Access each square with BOARD[Y,X] (yes, Y and X are reversed) You could use numbers to store which piece is on each spot, like 0=empty, 1=white pawn, 2=white rook, etc. (or use a string array and use the names of the pieces) And to fill the array with pieces at the start, you should use COPY:
COPY BOARD,@RESET @RESET DATA 2,3,4,5,6,4,3,2 DATA 1,1,1,1,1,1,1,1 DATA 0,0,0,0,0,0,0,0 DATA ...