No is hard to say

Today My friends ask me:"Hi Eric,do you want to come to our party tonight".but I readlly don't want to.I reply:"Thank you,but I have something to to"In this time,the word "No" is hard to say.how about you,when someone want you do something,and you really don't want to do,how to you say?"No,but thanks anyway" or just say "yes" instead,because of it's easy to say:)