myString[startPosition:endPosition]

package main

import "fmt"

func main() {

    myString := "Hello world"

    substringFromPosition := myString[1:] // starting from position
    betweenPositions := myString[1:5]     // between Positions
    tillCertainPosition := myString[:8]   // till ending position

    fmt.Printf("Starting from position substring %s \n", substringFromPosition)
    fmt.Printf("Between position substring %s \n", betweenPositions)
    fmt.Printf("Till certain position substring %s", tillCertainPosition)

}