How do I delete a background from my photo?
If you are working in Photoshop, which is not indicated, you must first select the part you want to save. Paste it and it will double in a new layer. Then you have several paths to go. You can place a new layer under the object layer. You can select the original layer and blur it to get rid of the details.
One of these two ways is what I would use. You can use the original layer, select the object, invert the selection and set something or blur the background.
Either way, be sure to duplicate the layer first and work on a copy so there is no chance that you can destroy the original.
What is basic knowledge in Photoshop?
Although there are hundreds of basic things to learn in Photoshop, layers and blending modes are one of the basic things that anyone should know before moving on to more complex ones.
When learning about layer blending modes, you can choose from hundreds of combinations which one suits you best.