Build an API

Hey guys, Ive got a desktop monitoring app and a mobile app and am looking for an API as link between both of them. What I want : Mobile app should create account via the new API API should include verification via either mail or Token as api-key(token = hash i create in my app and send to the API when signing up) User then inputs the clientid into the desktop software Desktopsoftware sends information to the API, mobile client is supossed to receive updates via API once a second or faster (realtime?) the mobile client should be able to relay some information to the desktop client too via API The Desktop-Client should be able to also send images to the mobile app (maybe multipart/form-data ?any solution is fine for me though)