Personally I started by reading Das Kapital (Marx), and it's really very good at giving you the foundations. It allows you to walk into any leftist space without feeling lost... basically it gives you a very good perspective and worldview

Later in my life after a few months I started reading Lenin... and it's great because he takes all the concepts you learned (or somewhat learned but didn't fully understand the first time) and reinforces them, giving you more insight. I started with The State and Revolution, and it worked for me

As Rindo said, State and Rev is a must.

"What is to be done?" (of course)

"Imperialism the Highest Stage of Capitalism"

and

"Left-Wing Communism an infantile disorder"

State and Revolution is a good start, equally great I'd say is Imperialism the Highest Stage of Capitalism. They're both very accessible.

You can read state & Revolution now.