Learn Dutch: Ik werk vandaag.

The basic, dictionary form of the verb (the infinitive) is werken (to work). To get the ik (I) form—also called the verbal stem—you simply remove the -en from the end. werken minus -en leaves us with werk.

Does Ik werk vandaag mean 'I work today' or 'I am working today'?

It means both! Unlike English, Dutch doesn't usually require a separate 'am working' form for things happening right now. You can use the simple present tense (Ik werk) for both 'I work' and 'I am working'.

What is the word order rule for a sentence like this?

In a standard Dutch main sentence, the verb must be in the second position. Here, the subject Ik is in position one, the verb werk is in position two, and the time word vandaag (today) follows it.