The first and most basic step is to create a new file. When a new file is created, you would be given options to customize the file and select the desired size. Though it is possible to change the options later when working on the file, it may affect the image.
It is advisable to choose the dimensions and resolution of the file according to where it is going to be used. While a larger file offers more details, it is also heavier and may take more time to download or upload.
Choose the desired color mode and if the background would be white or transparent. Each of them has their own advantages. It is easier to achieve most of the effects on a transparent background while the white one makes it easy to see the various steps of your work.