First off all don't panic. Any good instructor will make sure you can do these safely before throwing you into the lions den as it were. You will not be expected to know how to do them the second you walk into the dojo , all you can do is your best.
Saying that I still have trouble with them and tend to favour one side even if its not the side I am meant ot roll on
Make sure you listen to your instructor as they are the ones who are teaching you. And learn from others mistakes on forward rolls go over your shoulder instead of landing on your head and you will be fine.
Owie! I did that misstake when i did my first backroll. I got head stuck between mat and shoulders. I tried rolling straight backwards instead of over the shoulder. Damn that hurt.