In short — use arrow keys or "wasd" keys to move your character. Explore the map and find various places, like shops, gym, depot, arena, church, caves. To use bomb (when you are in gym, arena or caves), just press space bar.
About main places:
Shop: you will find items to your char, like armors, gloves, bombs and others.
Depot: here stay things that you have bought in shop.
Gym: kill little monsters, and increase your char with xp and coins.
Arena: battle with other people.
You can read detailed instructions by clicking on NPC's.