When I go for attack... How can I understand which side is best for start attack?... I mean how can I understand which side to I should drop my heroes?

If you are talking about a base, provided you don´t have heroes who pull the enemy like Ravager or Demon Slayer:

Drop your heroes as close as possible to the enemy hero you want to kill first.
Heroes like Renee Ven, Abyss Demon, Toxic Shaman or even Pan Goli are usually those heroes you want to kill fast.