Here's the FEN for the starting position:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
The 'w' indicates that it's white to play. Just change that to 'b'.
After the 8 slash-delimited lines that describe where all the pieces are, the 'w' or 'b' shows whose move it is, the KQkq stuff indicates where castling is still legal, and the 0 and 1 are counters for how many times the position has been repeated and how many moves since the last pawn move or capture (for the purpose of claiming draws).
Here's the FEN for the starting position:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
The 'w' indicates that it's white to play. Just change that to 'b'.
After the 8 slash-delimited lines that describe where all the pieces are, the 'w' or 'b' shows whose move it is, the KQkq stuff indicates where castling is still legal, and the 0 and 1 are counters for how many times the position has been repeated and how many moves since the last pawn move or capture (for the purpose of claiming draws).