Already, you've made it here. The final string to more "formally" know here is the thick E string; the 6th string.
When you're fretting the 6th string, remember, being a bit more of a "reach" with your fretting hand, good posture becomes important. If you want to review some notes on good fretting hand and body posture, this is an earlier blog post that covered how to hold the guitar.
The notes to know on the thick E string are the open note, the 1st fret's note, and the 3rd fret's note — those are the E, F, and G notes respectively. Notice how they're the same notes you learned on the 1st string (which was the thin E string)? They're just played a couple octaves apart, so just differing in pitch.
The first note, the E note, is played on the open 6th string.
Next, the F note is played on the first fret with your pointer finger.
Finally, the G note is played on the 3rd fret, with your ring finger.
Here's your exercise to workout the low E string's notes in your brain.
See you in the next few segments, which will cover a couple songs to apply the low E string.