Using money in Belgium ...
Belgium uses the EURO as does most of the other EU countries. The biggest problem you will face is the lack of business accepting credit cards. It is always best to carry cash with you and never plan on being able to use your credit card at local shops. The reason is most shops use a debit system called “Bank Contact” for transactions. Also, some places that accept Credit Cards require you to have a SIM chip (with pin code) embedded for you to be able to use it.
The cheapest way I found (for US citizens) to take out money while here is using a PayPal Debit Card. The card charges you only 1 USD to take out money and gives you a very good exchange rate. Plus the money in your account can earn good interest while siting in the account. Citi bank also has branches in Belgium and allows you to link your US account to a European account with free transfers, but I have yet to fully explore this option. If I remember correctly, Chase bank had free worldwide ATM transactions when I last lived in the US.
The best bank I have found for using your ATM/debit card to withdrawal money has been Fortis, but there are others which will work with your card. Ask your bank which international systems work with your card, then look for their logo on the window of Belgian bank branches.
You can also set up a Belgian account if you will be here for a while and get a debit card (bank contact card) for free. This might be the best option if you will be here for a while.
Money exchanging can be done at most banks around Belgium, but rates will vary, so do a bit of searching around.