Learning to float in water takes a lot of trust in yourself and in the person teaching you the skill. But, once you get into the habit of floating, there's almost nothing to it. But, take the matter seriously, dismiss criticism etc. You can do it!
Lie back in the water extending your legs and arms. Look towards the sky. Flap your feet and hands if you wish and perhaps this will keep you more comfortable and confident. Try not to get over anxious, relax as best you can. Have some confidence and (again) trust yourself. If at first you don't succeed, keep trying.
Confidence Builder:- Think about all the plonkers you know who can float easily - if they can do it, so can you.
Best of Luck.