If you do not know your size at all I suggest going to a shoe store (such as Footlocker, Champs, or Nike) and getting sized, try different sizes on to see what is comfortable to your liking. Bigger shoes will most likely slip off easily but too tight shoes can hurt your feet. Not good. 

If you are here because you are wondering "What size am I in Women's/Men's ?" Well the answer to your question is Men's and Women's sizes have a 1.5-2.0 difference. Example If i'm a size 7 in Women's I'm a size 5.0-5.5 in Men's or 8 in W I would be a 6.0-6.5 in M and so on... Now for Men wanting a Women's size you would go up two sizes, same concept. 

If you have smaller feet that don't fit into Women's I suggest getting Boys' GS size, it ranges from sizes starting at 3.5 (which is a Women's 6.5) and works it's way up and is different with every shoe.