The phrase "That which was from the beginning" refers to Jesus, who existed before everything was made. You could translate this as "We are writing to you about the one who existed before the creation of all things."
## the beginning ##
"the beginning of all things" or "the creation of the world"
## which we have heard ##
The phrase "which we have heard" refers to what Jesus taught them. It can be translated as "We heard him teach."
In verses 1 and 2, the word "we" refers to John and those who knew Jesus when he was on this earth, but it does not include the people John was writing to. (See: [[rc://en/ta/man/translate/figs-exclusive]])