App development

Hello everyone I am a self taught graphic designer(begginer of some sort,I look for advices and tutorials sometimes for learning new techniques,tips and tricks,etc) I want to create an app(mobile,or for both PC and Android) but I don't know where to really start. I want to know which apps are better for developing it,as for example is unity ok to create it?If not,which program I should use. Also,which types of apps are for begginer to learn the whole app development as a process? What type of app do you think it could be the most useful for you?