How do I make myself appear more approachable?

I have noticed a lot that I usually don’t ever end up getting approached by anyone when i’m out at parties or in public somewhere. A part of this is because i don’t like to let people of the opposite gender know that im paying attention to them. I really would like help on this, as well, but i have been single for 2 years and the situationships i’ve had since then have all ended horribly. because of this, i don’t like to go up and talk to people first. I have been told by my friends that i look like im always mad or disinterested and that i need to look more approachable. so how do i do that?