Another vote for Decathlon. The stuff is a similar price to supermarkets but a whole world apart in quality. Don't go near Asda, Sainsburys, Tesco etc if you want running gear, their kit just isnt up to it IME.
In Decathlon, head for the Kalenji range and get yourself a couple of pairs of Capris and some short sleeved t shirts.
You might want a long sleeved top for rainy days in winter, but I only wear mine when it's really cold - ie snow on the ground. I just run in my vest in the rain. I get hot and it's cooling!
You don't need a waterproof, you'll swelter in even the decent wicking ones. Only time I bother with mine is when the rules say I have to carry one in FRA races. Or if I'm heading out alone for 15+ miles in the hills and there's a chance I might get lost.
Once you've been running a bit longer and will be out for hours at a time you might want to invest in Ronhill or Gore stuff. When you do, Sportshoes online, as a pp said, is your best starting point. But honestly Decathlon is great for everyday basics.
The most important things,though, are decent running shoes and a sports bra. Good luck with c25k!