Something is wrong with my push lawnmower?

I was gonna mow the lawn so I checked gas and oil. I refilled the gas and then I put in new oil. I figured putting in new oil is same as putting in gas I added that then took it outside and tried starting it and the string you pull on wasn't working it would stick. Finally I got it started but it started smoking really bad so I turned it off looked at it and it had brown stuff leaking from it. I don't know what to do and need help?