It's confusing to say TO THINK in Portuguese. That's mostly because there are two verbs for it: PENSAR & ACHAR. Let's clear up the confusion with examples.
ACHAR or PENSAR? :: TO THINK in Portuguese
Most books will tell you that it's Ok to use either. That may be grammatically correct, but it does not fly in the real Brazil! You Need to start getting a sense that:
ACHAR: to think, to find
Use ACHAR whenever you could use "to find" just as well as "to think".
Use ACHAR in sentences like this:
Eu achei muito caro.
Você achou sua chave?
Eu sempre achei ela diferente.
Eu acho que é um TESLA.
Eu acho que eu te amo.
These would all sound a bit odd said using PENSAR instead of ACHAR.
PENSAR: to think
PENSAR is best used when talking about literally THINKING something.
Use PENSAR in sentences like this:
Eu sempre pensava no lado ruím das coisas.
O menino pensa muito bem.
Pense bem! Acabou a paciência.
Pensando melhor, eu prefiro comer pizza.
In all of these it would sound very strange if you were to use ACHAR in place of PENSAR.
The bottom line is this: While there are many cases where you could use either PENSAR or ACHAR, don't just use either. Instead, start listening to common uses of each in-context. That way when you have to make up a sentence in (ie: speaking!) you can visualize and recall these examples quickly.