[wrinkled courier]

Having decided I wanted to keep chickens I have spent most of the year reading on the subject.  I have proceeded with caution as I want to do it right and equally not throw money away.  I'm at the stage where I have built the coop and run, made my own feeders and waterers etc.  My question is, "What medicines and vitamins should I have on the shelf waiting for the arrival of the girls before I actually  purchase them?"

[joyfull]

you shouldn't need any vitamins - their layers pellets will provide all they need.
As for medicines some of the basic things you ought to keep are:- Flubenvet (used to worm chickens 3-4 times a year) purple spray (yes it is called that and you can find it in your local equestrian centres supplies), wound powder (this helps stop bleeding and is antiseptic), these three items are a must. On top of these are purely my own choice - avipro plus - I give this to my girls if they have had any antibiotics, critical care formula (I treated one of mine who had suffered a stroke with this), apple cider vinegar and either fresh crushed garlic or garlic granules (however do watch the amount of garlic as you don't want garlicky tasting eggs).
Some people use frontline to ensure you dont have mites (not to be confused with redmite) and as this hasn't been tested on poultry you should be purchased from your vets and signed for.

Antibiotics are obtained from your vets and try not to hoard these as they quickly go out of date when opened. Some have not been tested on poultry so you may have to watch how long you have to throw away the eggs.
Metacam, loxicom, meloxidyl are all anti inflammatory medicines and again are prescribed by your vets. Because these are normally used in canines your vet will need to work out the correct dosage.

[grinling]

Put a piccy of the setup so we can input ideas.
What chooks are you getting?

[snow white]

Ditto all the above.  But I do keep poultry spice for moulting time and when very cold in winter as it gives them a boost.