"I eat my lunch."
Translation:Ich esse mein Mittagessen.
GavChang, what do you mean, "nobody got an answer?" ? LingPenguin already answered the question perfectly. The chart dublinus posted was for definite articles ("the"). Indefinite articles ("a") and all other words ending in "ein" (such as "mein") work a little differently. Krueckauer clarified above what the correct endings should be.
You can see a table for "mein" here: http://german.morley-computing.co.uk/mein.php
The ending of the verb must match the subject. For a regular verb, such as "trinken," the endings are as follows:
I drink = ich trinke
you (singular, informal) drink = du trinkst
he/she/it drinks = er/sie/es trinkt
we drink = wir trinken
you (plural, informal) drink = ihr trinkt
you (formal, either singular or plural) drink = Sie trinken
they drink = sie trinken
"Essen" is just slightly irregular, but you'll see that the endings are still the same:
It's how the language works. English has the old form of "I'm breakfasting" = Ich frühstücke. Unfortunately, German decided to make Mittagessen and Abendessen seperable when you try to make them a verb: Ich esse zu Mittag & Ich esse zu Abend.
Du bist isst frühstückst is completely wrong because German doesn't apply English grammar rules to itself.