It’s correct to say “there are a lot of people”, since “people” is the plural of “person” (in this context, as it is most of the time). It would be wrong to say “there is” in your sentence, *but* the contraction “there’s” is often used in colloquial English even with plural subjects, so you will hear “there’s a lot of people”. You will even hear this from educated native speakers in informal conversation. Grammar fanatics are another matter….