Sentence Correction

I am having a difficulty understanding when to use TO as a preposition i.e. followed by a Gerund/Noun or when to use it as an Infinitive that is a simple verb.

eg. I used to drive is infinitve
I am used to driving is Prepostion,

but what prompts me to write driving instead of Drive when I add am in the sentence, what is the rule.

Thanks,

Vineet