Hello Sue,
Very unusual that a 237 goes out of time, check to make sure the needle is new and in the correct way round first, if you are sewing on any other fabric that cotton make sure you have the right type of needle.
If you need to retime it take off the needle plate and open the slide plate, tilt the machine back and take out the bobbin case, lower the gate down so you can see the half moon shape called the hook, turn the balance wheel towards you so the needle goes all the way down and then starts to go back up, the point on the hgook should be just above the eye of the needle, if it is not then you will need to slacken the screws on the right hand side of the long bar that goes from the hook to the right, you can now move the point of the hook to where it should be, tighten the screws back up and test sew, you may need to do this several times.
Ken
drivewithken, July 2009