Content not displaying correctly

Hello

I have made some changes in the css to main homepage and it has effected the content area within all other pages of my site.

The below link is to the about us page where this issue is evident

http://mikeswebsite.co.nf/?page_id=75

Someone has advised me that I have a negative margin on my “p” tags and I should paste the below code onto my CSS plugin/form/section:

p {
margin-botton:2em!important;
}

I know where to find my css files and to edit them but I am not 100% sure where exactly in the CSS style sheet I should paste this code.

I am fairly new to website coding / editing and any help would be much appriciated.

Thank you