Pokémon Diamond and Pearl/Lake Verity

Once you go to Lake Verity you will see a girl (or boy, depending on which gender you chose) and an old man standing and talking at the shore. They will thereafter exit, politely asking you to move aside so they may do so. Your rival notices that they left behind a bag of some kind. He says there's no harm in going into the grass for a second to see what's in the bag. That turns out to be wrong, as you are attacked by a, at level 2.

Starter Pokémon Choices
Watch the cutscene, and you'll be forced onto the tall grass, and into a battle. THIS IS WHERE YOU CHOOSE YOUR STARTER Choose whichever one you feel is the best or your favorite, as they will all help you in your journey. You should easily defeat the Starly with your new-found Pokémon.

The Battle
Keep slamming it with attacks 'till it's killed. Each of the starters start with a damaging move (Scratch, Tackle or Pound) and a stat reducing move (Growl or Leer). Starly should be defeated fairly easily.

After the Battle
After you defeat the Starly (and gain some Exp. Points) the young person you saw earlier will come back to fetch the briefcase, and sees that you took a Pokémon each. (S)he says something about it being "bad" and rushes off. You will exit the lake front and walk up to the girl (Dawn) or boy (Lucas) and the old man (Prof. Rowan). Some dialogue will go back and forth, ending with an invitation back to Prof. Rowan's lab if you have the chance. You'll get that chance very soon, but for now head back home to your mother and get healed.