Go through your wardrobe and put clothes into piles by category (tops, skirts etc). Then sort what you want to keep, send to charity, ebay or throw out because they're worn out. Then you can make a list of what you need from what's left over. Do you want a completely new look or just to add to what you've already got?
Have a look on pintrest and fashion magazines for styles that you like and go shopping. Zara, H&M, Mango, NewLook are all good for basics such as t shirts, jeans etc. What's your life style like as that will influence the types of clothes that you'll buy.