Once you have the funds in your wallet, please visit a proper market. You need to find a trading pair of a currency you presently have and the one you want to obtain.
All available trading pairs can be found here: Markets
Use a search bar to find available pairs for your currency.
There are two ways to exchange your funds: you can either place a new order on the market and wait until other users like it and accept (fully or partially) or you can choose from already existing offers placed by other users. By choosing the latter option, you will exchange your funds in a matter of seconds! By choosing the former option, be aware that your funds will become reserved (not available for you) as long as your offer is open.
Always remember to analyze the market before placing your own order. If you set an unrealistic price, your order may never be realized. But you can always cancel your unfinished orders and create new ones here: My orders
The process of creating or accepting offers is the same for all pairs. Here is an instruction of buying BTC when having EUR funds.
Visit the EUR/BTC market here: BTC/EUR market
Analyze the market and chart. More on reading charts can be found here: Reading market charts
Decide whether you want to place an order or accept an existing one.
For placing an order:
put up an offer of your own in the Buy Box on the left by filling in the fields. You will see three fields: price (set the desired price in EUR per 1BTC but stay savvy), amount (of BTC you want to buy) and the total price of your offer will be automatically calculated. Check whether you have enough funds for realizing it. After all of the fields in the box are filled the way you wish, click "Buy" to put up an offer on the market.
Once your buy offer finds a matching sell offer, the transaction will be automatically executed. Remember that is it possible for only part of your funds to be exchanged for the desired currency. Another user can sell you less BTC than you wanted to buy, but for the same price per 1 BTC. Your offer will be active on the market until it will be fully realized or until you will cancel the offer.
For accepting an order:
click on an existing offer in the "Sell orders" section (marked by red color). The ones at the top of the column are the most profitable from a BTC buyer's perspective. When you click the offer, all the information will be automatically filled in the box on the left side of the market. There you can modify either the price or the amount. After all of the fields in the box are filled the way you wish, click "Buy" and your transaction will be automatically executed.