How to delete a folder of photos on my iphone 4S?

I just got an iphone and i had a folder of photos on my computer that I wanted to put on it but know because it goes on the iphone as a folder but because that's all the photos i have on it so know i have two folders with the same photos in them so how do i delete the folder i put on them and not the photos in it so i only have the photo library? Please help thanks

Added (1). Thanks for the help but there isn't a bustbin icon or a red circle when i press delete

There's a little dustbin icon that you tap on

Press the edit button on top right hand and then red circle and then delete