"Let's get started" sounds the most natural. you can say "let's start" or "let's start now" as well. "Here we go" is a bit more informal, and is something you would say almost to yourself rather than to a group of people. "let's get it started" is actually very informal and has the feeling of teenager talk... it refers to something ("it") which is left unsaid... example: "let's get (the party) started", or "let's get (the dance) started". It can also refer to sexual innuendo.

Yes it is if you don't forget the apostophe. When you want to start do something with someone how do you say it in English? Let's get started Let's get it started Let's start Here we go what are the differences? No difference

這些差不多。 "let's begin" 比 “let's start” 更慣用。

It depends on the context. All you have suggested are correct. As well as let's begin let's go