I am trying to implement a step by step sign up process for my app, like the ones seen on Facebook (mobile app) and Twitter. My question is, once the user clicks on the sign up button should I display any welcoming message ( for example, Welcome to my app! It only takes a few minutes to sign up) or should I just start the sign up questions without any messages?
Whether you show a welcome message really depends on the tone you are trying to set for your application. If it's a friendly, chatty application, then a welcome message could set that tone straight away, and set future expectations for interacting with the app. If it's a formal business application, then a welcome message may not be necessary or appropriate.
If you do want to show a message, I would recommend showing it either on the same page as the Sign Up button, or on the first page (ideally the only page) of the Sign Up form. In other words, don't make users click through a welcome message just to get to the first proper step of the task.