The 26-year-old United States international moved to Deepdale on loan for the remainder of the season from Premier League Fulham.
"The bottom line is I've come to score goals," he told BBC Radio Lancashire.
"I like to stretch the back four and use my pace to get in behind defenders and hold the ball up."
He added: "I'm adjusted to English football and now I can show my worth and I couldn't ask for a better opportunity for a team who needs a guy to come in and score goals."
Johnson admits he found space at Fulham difficult to come by.
"I was at home with my wife and new baby, and I got a call from my agent saying, 'Would you be interested in going to Preston?'
"I said, 'Let me talk to my wife first to see how she feels about me being away'. She said, 'This is what you've been waiting for.'
"So I spoke to my agent and said, 'Yes, I think it's a perfect move for me if Fulham's willing to let me go'."