How do you start a lawn mower after winter?

I have a small, 5HP push mower and I had it running a couple days ago, but now when I try starting it, it would start for a brief 3-4 seconds top, and then stall. I don't know what the problem could be, my spark plug looks like it was in good shape, and I tried cleaning any grass out of the air filter, but now I think the problem lies in the gas or oil. Do I need to change a oil filter? I just put new oil in after mowing my lawn the two days ago, but when I checked the oil foil/wand the oil was pretty dark and gritty....... I ran the winter gas tank down as much as I could and I put fresh gas in it, which is good, how do I fix this?