Messages should come in a standard format, you can check what that is in CAP413. Most messages will be very predictable with only a few changes, so concentrate on the bits that are different.
When you start up and ask for aerodrome information you will be given the pressures, you can measure these in advance with you own altimeter so they should not surprise you.
Most important is to always readback information in the order it was given, that way you have a better chance of remembering it. If you reverse the order you will nearly always forget the first bit.