You need to dress the way you did for your interview or think back about how people in the office were dressed and emulate them.
The first thing you need to decide is if you plan to dress up in leggings, trousers or a skirt for work. Once you decide this you can begin to build your wardrobe. Essentially you will need a good suit, 3 to 4 blouses, a jacket styled sweater and shoes to match these outfits.
Buy blouses to match your suit and trousers. Pick up blouses and jackets in your favorite color so that you are happy when you are in your clothes. Tops need to be a shade darker than the basic color of your suit if you opt for a lighter shade for your basics.
Sort out these clothes from the rest of your daily wear. Keep your flip flops and jeans and T shirts aside for the moment as you will need them only over the weekend.
Now go through the formal clothes that you have and attempt to create a look with these clothes.Mix and match works well to in any formal set up so that you have the liberty of pairing a pair of black trousers with a red blouse or a beige skirt with a brown top.
If you are clueless about what to wear then browse through a fashion magazine to figure out what suits you best based on your figure and job. You could also visit a store where they have displays of formal clothes put up. These are two reliable sources that you can depend on for your fashion queries.
You also need to have a few pairs of formal shoes. Include a pair of basic pumps (black or tan) in your wardrobe. You also need to have a good handbag to carry all your essentials around. Having a watch on your wrist is considered to be extremely professional and you may want to sport one from your first day at work.