In Brazil:
>Achar: Means to find somenthing or to think something without sure.
>Pensar: Means to think in something or express an opinion.
But, depends of the context:
When "achar" means "think":
>Where is my keys? --- Onde estão minhas chaves?
>I THINK that you forgot in somewhere. --- Eu ACHO que você esqueceu em algum lugar.
In that case, express that what you think about something, but you not sure about this.
When "pensar" means "think":
>What do you THINK about policy? --- O que você PENSA sobre política?
> Well, I THINK that the policy in our country is organized and fair. ---Bom, eu PENSO que a política em nosso país é organizada e justa.
In that case, "think" express an opinion that you have sure.
Simpler than this, "achar" means "guess":
>Do you THINK will it rain today? ---Você ACHA que vai chover hoje?
>Yes, I GUESS. ---- Sim, eu ACHO.
When "achar" means "to find"
> I need TO FIND my keys. --- Eu preciso ACHAR minhas chaves.
I hope that helped you.