Good Jumps for beginner Jumpers?

Hi, I am borrowing my friend Johanna's yahoo account, my name is Kayla, Johanna is letting my jump her horse Jethro tomorrow, I have been riding for a few months, she has a series of jumps, and she told me to choose something I looked comfortable with and she'll evaluate if it is safe for me to jump it, how high should I jump? Also what does it feel like to jump the first time?