Which birth control should I use?

I've been having sex with one person in particular quite a lot lately and I've been thinking that it's time for me to start using some sort of birth control.
When I've been to get the morning after pill, they always make the implant sound like the best option. My only problem with it is that I've been told that it makes you lose your sex drive.. Is this true? 
I've also thought about the pill but I'm worried that it would make me gain weight like crazy.
I'm just not sure what would be best for me. 
Opinions?