Stalling, not starting

Sometimes my van won't stay running after I start it unless I rev up on the gas peddle, as well as sometimes when Im in drive but stopped with my foot on the brake(like if I was at a curb waiting for someone) when I let off the brake to pull away it will stall out, also sometimes stalls out or acts like it wants to stall out when I put it in reverse and start to back up, as well as it sometimes stalls out or acts like it wants to stall out when I put it in drive and start to take off, and lastly sometimes I have to put the gas peddle to the floor to get it to start. I wanted to know what the problem is and how to fix it, thanks.