Again this is not really an SFML question, but is a general game logic / math question.
To "look at" another object, you'll need to know the angle of rotation. You can find that with the atan2 function in the <cmath> header.
Though note a few things.
1) SFML has Y coord inverted from normal (+Y is 'down' instead of the usual 'up')
2) SFML expects angles in degrees, whereas atan2 gives you the angle in radians.
1 2 3 4 5 6 7 8
double angle = atan2(
my_pos.y - target_pos.y, // SFML has inverted Y
target_pos.x - my_pos.x );
// convert that angle from radians to degrees
angle *= 180.0 / pi;
// give that angle to SFML with setRotation or whatever the function call is