"It rains a lot."
No, llueve = it rains. What you wrote would be something like "It rains it a lot," which wouldn't make sense in English nor Spanish.
I was trying to use lo as "it", but I guess "it" is just assumed and the way I had "it" the lo would be reflexive. and wrong.
Right, "lo" is a direct object pronoun, never a subject pronoun. There is no subject pronoun "it" in Spanish (not used commonly anyway), since it is always implied as part of the verb.
Not sure if it is grammatically correct, but it would be redundant either way since llueve tells you its third person he/she/it, where he and she make no sense, so it is it :) You might be able to say INF + Pronoun like Lloverlo mucho, but not sure