The residents in the Southern portion of Belgium (Wallonia) speak French.  The residents in the Northern portion (Flanders) speak Flemish, which is the same language as Dutch.  About 10,000 people living on the Eastern side of Belgium (cities of Eupen, Malmedy, St Vith) speak German. The three official languages are therefore French, Dutch and German.

Brussels is somewhat 1/2 & 1/2, very few people speak 'Brusseils' (a mix of French and Dutch - you are likely to find them in the neighborhood Les Marolles) and many people in Brussels speak English.

The Flemish are likely to try and switch to English as soon as they notice you're a foreigner. In general, they have a much better knowledge of English than the French speaking people. Language is a sensitive topic in Belgium. Do NOT use French in the northern (better organised, cleaner, larger and richer) part of the country, but stick to English or try some Dutch. 

Important Phrases in French 

-Hello: Bonjour

-Good evening: Bonsoir 

-Bye bye: Au revoir 

-Thank You: Merci

-Please: S'il vous plait

-Excuse me: Excusez-moi

-I am looking for...: Je cherche...

-Where is....: Ou se trouve...? (the nearest ATM: le bancontact le plus proche)

-How much...: Combien coute...? 

-French Fries with Mayonnaise, please: Une frite-mayonnaise, s'il vous plait!

-My name is: Je m'appelle...

-I am from: Je viens de...

-I am lost: Je suis perdu

Important Phrases in Dutch

-Hello: Goeiedag

-Good evening: Goeden avond

-Bye bye: tot ziens 

-Thank You: Dank u wel

-Please: alstublieft

-Excuse me: sorry

-Where is....: Waar is...? (the nearest ATM: een bancontact)

-How much...: Hoeveel kost ....? 

-French Fries with Mayonnaise, please: frietjes met mayonnaise, alstublieft!

-My name is: Ik heet.../Mijn naam is...

-I am from: Ik ben van...