It is just like non-standard German:
I am eating. - Ik ben aan het eten.
There are also other ways to express it using "liggen", "lopen", "staan" and "zitten"
- I am reading. - Ik lig te lezen. (lying down and reading is implied)
- I am singing. - Ik loop te zingen. (walking and singing)
- I am talking. - Ik sta te praten. (standing while talking)
- I am sleeping. - Ik zit te slapen. (sitting and sleeping)
That is really confusing, I know :(. They are both definite articles, meaning 'the'. Some words go with 'the' and others go with 'het'. You'll find some more help here: https://www.duolingo.com/skill/dn/Basics-1/tips-and-notes. You will have to learn them by heart.