Learn Polish: Chcę trochę cukru. - I want a little sugar.

The word trochę (a little, some) is a quantifier. In Polish, quantifiers force the noun that follows them to take the Genitive case. So, cukier changes to its Genitive form, cukru.

Doesn't the verb chcę usually take the Accusative case?

Yes, the verb chcieć (to want) normally takes the Accusative case for the object you want. However, the quantifier trochę is the boss here. Words like trochę, dużo (a lot), and mało (a little) always force the following noun into the Genitive case, overriding the verb's usual rules.

Where did the 'e' go when cukier changed to cukru?

This is an example of the fleeting 'e' in Polish. Many masculine nouns ending in '-ier' or '-ec' drop that 'e' when a grammatical ending is added. So, cukier loses the 'e' before attaching the Genitive '-u' ending.

Why is there no word for 'I' in this sentence?

Polish often drops subject pronouns like ja (I). The verb ending already tells us who is doing the action. The '-ę' ending on chcę strictly means 'I want', so adding ja is unnecessary unless you want to add special emphasis.