I'll throw my 2 cents in

I started riding more a year ago and faced purchasing all the "stuff" to do a supported tour. It is pricey but I did it increments and things don't wear out quickly

You have a helmet I am assuming is OK

Clothing:
#1 Bike shorts, I would definately make this my first purchase. for long rides on consecutive days I found nothing beats the pearl izumi touring chamois'.
2 pairs if possible in case pair #1 doesn't dry quickly enough

#2 - gloves

#3 wicking top of some kind - I bought some inexpensive workout ones at target, but also have some jerseys - I like them both

#4to stay warmer I just use an old pair of unlined wind pants (they are older than the hills, pack small, can be taken off w/o removing my shoes and work great.
On top I just wear a wind breaker of some type that doesn't flap alot

#5 Socks N/A I am one of the few who goes barefoot in my road shoes. In my mtb shoes that I wear in the fall I wear socks but then I wear warmer ones just to keep my feet warm. I wouldn' t put special socks high on the list yet because there are a lot of other options - BUT cycling socks are cute - great Stocking stuffers, mothers day gifts etc etc etc.

Bike

Absolutely necessary
#1 spare tube, patch kit, levers, something to fill it with (a lot of women go w/ the co2 kits) People will usually help you change it and if you have your own stuff you are thought much more highly of - most won't want to give up their spare tube and no guarantee it will fit.

#2 Lube - learn how to use and how often - some need to be applied every 50 or 60 miles

I agree w/ all that said a floor pump for at home

Work up to it gradually and eventually you will have what you need
(I worked temp jobs and earmarked the extra cash for bike stuff only