There is a ton of ways of taking a picture of the screen and save it to an image file. You may use special applications or services, but let's see how it can be done with a preinstalled Windows 10 software. Here is a step by step guide of how to do this.
Run an application you are going to take a screenshot from. Prepare it like before photoshot. I will use a calculator as an example.
Press Ctrl + Print Screen button to make a screenshot and save it to a clipboard memory.
Now you need to save screenshot to some image file. Let's use default Windows 10 image editor called Paint. Press Windows key + S to open search box.
Type Paint to find preinstalled Paint image editor app.
Press Enter to run it.
Create a new file using the menu File → New or press Ctrl + N.
Paste a screenshot from the clipboard by using the tab Home → Paste or press Ctrl + V. Now you have the whole screen inside a new file. But if you want just the part with app's window you need to crop it.
Activate selection tool by using the tab Home → Select.
Click on the image in the upper left point of the app's window where you want to start cropping and drag mouse to right bottom corner of target window inside screenshot. Release left mouse button to finish select proccess.
You can't tweak a selection rectangle after releasing left mouse button, so be accurate.
If you made a mistake and selected a wrong piece of the picture, just start again. Old selection will be reseted.
When you selected a correct area click on the tab Home → Crop (top icon on the right near Select tool) or press Ctrl + Shift + X.
If you want to revert the crop action press Ctrl + Z.
When you are ready to save a screenshot go to the menu File → Save as → PNG picture or JPEG picture. Use png format if you don't want to lose quality and you don't care about the size of the image. Png format may produce large size files for photos, but it is very useful for texts and images with limited color palette. Use jpeg format for photos or if you don't care about the quality.
Type file name and click on Save or press Enter key.
Now you have the screenshot saved in the image file ready to be send by email or messanger. There are a lot of other ways to share your desktop, but you need to install third party software to do so.