No creo que + subjuntivo (cuando tenemos duda)
- Creo que hace calor aquí.
- No creo que haga calor aquí.
But I don't understand this sentence:
Creo que no hace calor aqui.
All that matters is whether the main clause is one of belief or non-belief. So a negative belief requires a subjunctive in the subordinate clause, no matter whether the subordinate clause is itself positively or negatively expressed.
(But avoid that last one if possible, as it is potentially confusing.)
So the mode of the subordinate clause has nothing to do with its own positivity or negativity. That said, there are scenarios that admit both, and you can tell by which one the speaker selects how certain they are of the matter:
In the second, the speaker is expressing their own lack of confidence in the questioned outcome.
The verb dudar works mostly opposite of creer in its choice of mode, taking now the subjunctive in the positive but (sometimes) the indicative in the negative:
However, with dudar there is an increased tendency to use the subjunctive even when dudar is itself negated. To not do so, as I have just done above, means that the speaker is quite sure that there can be no doubt that this is certainly fact.