"I think it will rain today."
Translation:Pienso que lloverá hoy.
When you use "ir + a + infinitive" as a future, you need the "a" after the conjugated ir.
I believe that "Creo que" means "I believe that". (a bit of the play on the words) In a casual way, I would say either of "believe" and "think" in an English sentence to indicate the same thought in this context. I just discovered something called The Visual Thesaurus today, (probably been lying around like a snake in the weeds for a while;)). I may test it on these words. Best regards to you.