I am no style maven but a relatively stylish and also very casual dresser. I found pinterest useful for this, just search spring/summer/fall casual outfits and if you see something you like add it to your board. Also look at hush website for ideas on putting outfits together, they have a great laidback casual look.
Another (boring but practical tip) is try to buy the majority in colours that go together so you can mix and match, most of my day to day stuff is: navy, olive, tan, grey, white and denim with a few coral/pinky/blush pieces. Going out tops don't have to be as flexible so you can vary the colours or pattern a bit more on those.
I would buy (not all at once)...you probably have some of this already...
Trousers
Smart dark jeans maybe straight leg
Girlfriend/casual jeans in lighter wash
skinny jeans in medium-wash which can be rolled up too
white jeans
olive green/dark grey joggers
A nice casual skirt, which can be dressed up or down
Shorts for summer if you like wearing them
Tops
V-neck or scoop neck t-shirts; navy, olive, blush, stripy etc
long sleeve or 3/4 bretons in different patterns
Maybe a boho blouse (nothing too floaty/wide)
few dressier tops for going out
Some thick cotton vest tops for summer (nothing too flimsy)
Shoes
Classic trainers such as adidas or converse
low-heeled suede ankle boots - dark grey is very versatile but taupe is nice for spring
Ballet shoes in a nice neutral nude or leopard print
Leather sandals for summer in tan or black
Jackets
Denim jacket
Utility type jacket in olive
Casual blazer
Bags
A nice crossbody bag in navy or tan
A dressier clutch for going out
Scarves
A couple of new ones maybe look at hush
Accessories
Buy a few decent pieces; such as pendant necklace, bangles, nice watch, nice sunglasses
All this stuff is very basic and easy to put together.
olive jacket + girlfriend jeans + navy top + trainers
or
Denim jacket + olive joggers + blush top + sandals
or
blazer +white jeans+breton+ballet shoes
just add accessories and you are done :)
Happy shopping!