There is girl i have promised to have sex for a while ,i,v been knowing her for years and she thinks alot from me

problem is she is expecting me to satisfy her but i have realy noe idea what to do in bed because my first girlfriend use to mark me because of my bad bedskills and ever sense when I.v been having sex it is just got worst.I was wondering how she would feel about it if I tell her that ,but then again its a bad idea because all she wants from me is a good sex.She said it byherself.
Sorry for my bad english
