My css links formatting is interfering with that of Bootstrap. How do I prevent this?

I am learning how to use Bootstrap and I know the idea that i should be including Bootstrap first and then my own stylesheet. The issue is that, i have my own formats for ul, li a.active etc but Bootstrap also got them. For the header section, i want to use my formats but after I want to use those of Bootstrap. They are now interfering. How can i do this?