Either "Let's get started." or "Let's go into more detail."

To piggy back on Michael Ʃ's example of "start your engines," to say the opposite, sometimes people say "cool your jets," (though you don't hear that too much nowadays) or "hold your horses." Both mean "slow down" or "calm down" https://www.urbandictionary.com/define.php?term=cool%20your%20jets

When I hear "let's get into it" it makes me think of a couple of different things. The exact meaning would be contextual. - Let's get into something: let's have a go at surfing, for example. It would be great, we might really enjoy it and I'd love to get into it. - Let's get into a subject: Let's 'hash this out' and have a deep discussion/argument about something. Let's get it done and deal with it. - Let's get on it: Let's start something, let's get going, let's start doing the thing we talked about.

"Let's get started." More flippantly, we might think: "Here we go." "Ladies and gentlemen, start your engines. " https://www.phrases.org.uk/bulletin_board/29/messages/942.html https://www.merriam-webster.com/dictionary/here%20we%20go