Step 1 : Take a deep breath in, hold it in for 10 seconds, and slowly breathe out...
Step 2 : Sit down and close your eyes
Step 3 : Repeat step 1 two or three times, then open your eyes (if you're still awake)
Now you're ready, to take your next steps.
Your PPL training will have encompassed all the things that apply to most SEP (single engine piston) non-complex aeroplanes. All planes are ever so slightly different, speeds vary slightly, the view from outside the plane is ever so slightly different too - even in the same type of plane, two C150 may feel completely different to one another!
Biggest differences between the C150 and C172 is that the the C172 is much bigger, has 4 seats, more fuel, and generally a bigger engine which consumes more fuel per hour. The biggest difference to me was the weight of the plane, was that the 150 really gets bumped around in every thermal / gust, whilst the 172 is a little more stable. Depending on what year / type / upgrade has been done on the 172, you may have a fuel injected engine which although runs the same, has a different start procedure and no carburettor heat which you'll need to get used to. But all the main buttons and knobs are all in the same place.
What I would do, is ask the school where you are going's POH for the C172, and skim read it paying attention to the weight and balance, various speeds (landing / takeoff / stall / vfe....), any limiting factors (such as VFE), takeoff and landing distances, and fuel contents. The school may also have handling notes, that may be of use to you. Also ask if they have a checklist that you can buy / have - that will get you used to the checks required (its all generally the same between the two, but familiarity will help!)
On the whole, it isn't THAT different - and you'll be just fine! Don't expect to be a perfect pilot within a 30 minute check flight - but after a few hours, you'll realise that its all the same!
Hope this helps!