You need to practice on some much smaller trees first. I wouldn't touch anything over 25" with an 18" bar personally. There is a fine art to felling a tree, and felling it correctly. We cut a lot of wood each year for personal use and selling.
What species of tree are you cutting that large? How is it leaning? Is it dead? Lots to questions to ask before you get into a situation where things could become life altering real fast....