Yes, you could monetize your apps built on Salesforce, but you need to follow a pricing model from Salesforce, in which you need to pay some set amount to Salesforce.
Want to become Salesforce Certified Administrator, to make progress in your career, then I would recommend you to take up the Salesforce training from Intellipaat which is a 36hrs online training course with 52hrs of project work and exercises to gain proficiency in Salesforce.
Also, check out our YouTube video on Salesforce custom apps to get started.