Find the Four Flying Flea Friends and buy a baseball bat, a Dirty Sock and as many Health Vials as possible.
Now talk to the smallest Flea and beat the mini game for an achievement.
Frogbert's house is at the North of River Town. Egbert's is South of the Flea Friends. That's two more achievements.
You're now ready to pursue the ants. Head to the South exit of River Town. You'll wind up on the map. Go South to the Intersection area, then head West to the Any Colony.