I need to promote my website. And so that it is in the top of search engines on the first pages. What are the most effective ways to promote today? I've heard about the backlinks method. How effective is it? How does it work? Please tell me.

How does application development work and how does development affect the cost of the application? What does it depend on? I want to develop my own application and I don't know if I have enough budget. Is there any way to calculate the cost of developing an application in advance?