A Salesforce developer is an expert who works on building applications in the Salesforce platform using various tools. There are resources available in the market like books, pdf, online tutorial videos, and so on. These all just provide you with theoretical knowledge which will be useful in some instances. Better go for instructor-led training where you get a chance to learn under the guidance of experts. From this, you will get both theoretical knowledge along with practical knowledge. For that, I will recommend you this salesforce certification course from Intellipaat. This training provides all the required skills and helps you to become a certified salesforce developer.
Before that do check out the video on how Rohit Bhargava became a salesforce developer with the help of Intellipaat training-