pensar vs creer
Both translate as 'to think' — pensar is active thinking or planning, creer is holding a belief.
pensar
To think actively — reasoning, planning, having opinions about something.
¿Qué piensas de esto?
What do you think about this?
Asking for an opinion or reaction. Pensar en = thinking about; pensar de = thinking of (opinion).
Pienso ir mañana.
I'm thinking of going tomorrow. / I plan to go.
Pensar + infinitive = to plan or intend. Very common. Not creer.
Estoy pensando en ti.
I'm thinking about you.
Active mental occupation. Pensar en = to think about (someone/something).
creer
To believe, to think in the sense of holding a view or assumption.
Creo que tiene razón.
I think he's right.
Expressing a belief or opinion. The most natural 'I think' in conversation.
No creo que sea buena idea.
I don't think it's a good idea.
Negated creer triggers subjunctive. One of the most reliable subjunctive triggers.
¿Tú crees?
Do you think so? / Really?
Conversational skepticism or interest. Extremely common filler response.
Tricky bit
'Pienso que' and 'creo que' both work for opinions, but 'pienso que no' (I don't think so) is less common than 'creo que no.' And 'pienso en ir' (I'm considering going) vs 'creo que iré' (I think I'll go) — subtle but real.