My first hill was Ben Nevis, 20 years ago- I didn't know what it was, I just woke up in the tent at the bottom (Glen Nevis campsite) and watched the tiny people coming down this amazing looking hill, and said 'I HAVE TO DO THAT!' I did it the next day and was lucky, with a superb view. I was living in the mountain-free Northern Isles until recently, which made it hard to reach the hills very often, but I'm in a better location now.
I always take far too much stuff, but it's better to be over-prepared than missing something vital - for (non snowy) hikes in the gentler seasons (I don't do winter conditions) I normally take the following (waiting to people to laugh)
good boots/socks (number one!!)
stretchy trousers or leggings NOT jeans
hat (I have a Tilley with a big brim)
a few layers on top
waterproof jacket
wooly hat, gloves, scarf (even in summer)
more water than you think you'll need, unless there are water sources you're happy with
food
map/compass (and know how to use them)
walking poles (helps me a lot especially coming down)
torch
a mirror (in case you get a midge in your eye!)
midge repellant (don't put it in your eyes!)
tick removal card (never needed yet)
sunglasses/sunscreen (once forgot sunscreen and had a tan stripe across my head from my headband ALL SUMMER)
crocs if I'm expecting a river crossing
phone, fully charged
camera (sometimes just use the phone)
seat pad thing for lunch time in case of soggy ground
aspirin in case of a headache, which I sometimes get in hot weather
Bea, my bee who comes up hills with me
I'm also not keen on climbing hills if it isn't fine weather, as I take loads of photos, and being soaked and battered and getting no view isn't my idea of fun, so pick a good day - it's worth waiting for!