In the context of crowdsourcing communities (e.g., Citizen Science), crowd engagement is a significant determinant of projects' sustainability. The challenging missions of finding motivated people to participate in such initiatives and triggering their engagement to the cause have been widely acknowledged by scholars in the field of Citizen Science (Eveleigh et al., 2014; Nov et al., 2011b; Raddick et al., 2010; Rotman, 2013; Rotman et al., 2014); both on crowdsourcing initiatives (Balestra et al., 2017; Lampe et al., 2010; K. Y. Lin & Lu, 2011; Preece & Shneiderman, 2009; Steinmacher et al., 2015) and on online communities, in general (Brabham, 2010; Crowston & Fagnot, 2008; de Vreede et al., 2013; Zheng et al., 2011). The initial interaction with the technology employed by crowdsourcing platforms, including Citizen Science, affects users' experiences and should be designed considering their effects on the initial engagement. This work focuses on understanding how onboarding impacts early engagement and, consequently, the likelihood of boosting the quality of initial interaction and sustaining the adoption. Early engagement means the intricate process of embracing users' characteristics and motivations during the first interaction. The goal of Citizen Science platforms when implementing an onboarding design is, in general, to turn first-time visitors into a long-term users by scaffolding the first use toward participation. The central premise of this investigation is that onboarding characteristics and users' initial experiences largely determine whether they ultimately continue using the app; therefore, the thoughtful design of the first experience is fundamental. Organized in eight chapters, this doctoral dissertation starts by offering insights into the variables involved in the process of onboarding new users. Although commonly employed by the SaaS industry in various applications, onboarding design still lacks systematic investigation and precise definitions. Therefore, this research presents a terminology for the onboarding process and defines its four structural elements: Statement of Purpose, User Identification, Informational Support, and Conversion Event. Delving into the Citizen Science context, it is conducted three studies on how existing projects employ onboarding practices in their mobile applications. The studies, in chapters four to six, reveal barriers and reactions to onboarding experiences from volunteers. For example, making the statement of purpose clear, explicitly showing why individuals should be volunteering, and being part of a contributing crowd, apps have promising chances of keeping users engaged and returning in the future. Through various analyses and discussions, this work provides novel comprehension of how first-time interactions have the potential to alter newcomers' engagement in mobile apps. Finally, this investigation offers guidelines to support the designing decision process of creating a successful onboarding flow, primarily in the Citizen Science domain. It is presented seven drivers of newcomers' engagement that consist of design recommendations for onboarding that can be adopted for virtually any crowdsourcing app. Key drivers include essential concerns that influence engagement and can be resolved, for instance, by providing information on the users' roles and their contributions to the project, plus informing the app's goals and impact on the world with transparency. The seven drivers address cautious use of (1) technical language and jargon; to encourage (2) informing users regarding app's mechanics, and offering guidance to tasks accomplishments; stressing (3) the users' roles and their contributions' purpose within the project; to be transparent about the (4) app's goals, results, and impacts on the world; elucidate any (5) benefits or rewards right from the beginning, even they are not tangible or immediate; consider (6) UI's visual quality as a decisive interest factor and design it according to the intended audience; and lastly, (7) to advise the use of visual cues to enhance usability and reduce uncertainty. This dissertation has a pivotal contribution: the definition of terms and operationalization of onboarding elements, their attributes, and roles upon users' needs and individual aspects. Moreover, an onboarding flow creates an opportunity to successfully captivate and retain newcomers only when design and engagement attributes address users' characteristics, needs, and motivations.