Sports direct always is cheapest in my opinion. I always get pants with a zip pocket and a tie waist (but tie it before you put on or you lose the strings!). A few good bras and cheapo Tshirts with a high neck so you don't fall out when your 'planking'. I run a lot so I have a few zip up tops with zip pockets as if I put my keys an phone in my pants pocket they fall down but you won't need that in the gym. If you seriously intend to run you need proper running trainers - sports
Shoes.com are good, for anything else just a bog standArd trainer will do and sports direct is the one for you.
Just don't spend loads if you expect to lose weight! Oh and dark pants - a sweaty bum isn't always that attractive!