Hello, don't worry - you will have a lovely time although it won't be as relaxing as holidays pre-baby!
We went to Spain when dd was 4 months old and have recently been to France (she is now 1) - it was harder work this time!
Check with the airline about baby equipment - with easy jet we had an allowance for baby equipment and took the travel cot and pushchair plus an extra case for her. You are allowed to keep the pushchair till you get to the plane.
Don't bother taking loads of nappies/ baby wipes - you can buy them there.
Remember baby sun screen and calpol!
Don't know about formula/ bottle feeding as dd was fully breastfed when we went to Spain, so that was easy.
I always breastfeed dd on the plane and she always passes out unconscious - think she finds the engine noise soothing! Babies suffer with ears popping on planes so it is a good idea to have them suck breast/ bottle/ dummy.
You can take bottle/ beaker on the plane. Security will make you taste the contents!
Take favourite toys and fun things for the pool/ beach. We bought an inflatable there.
Take sun hats/ sunshades.
We took DD out and about with us in the evenings - she slept in her pram.
Ummm - that's all I can think of at the moment.
Oh, remember people are very kind and helpful when you are travelling with a baby and the Spanish love them and make a gratifying fuss!!