I am a new user. I am new to the operation and navigation on Krita. I don't know whether to bring out the color wheel for picking color. I would like to know what kind of brushes are good for coloring. If I save a drawing on Krita, can I export it to my cell phone, Whatsapp or email?

Thank you in advance for your advice

If you want to learn the details of how to use krita, the manual is here: https://docs.krita.org/en/
Also, there are many tutorial videos on YouTube, made by many different people. I suggest that you search for 'krita tutorial' there.

When you do any work on an image, you should always Save it as a .kra file, this is krita's own file format/extension and that file is capable of storing every aspect of your work, such as layers, transparency, filters etc, not all of which can be stored in other file formats such as .jpg and .png.

If you want to make images to put on your phone, a website, etc then within krita you can Export your image as .png, .jpg .gif, .bmp etc and this will make a separate image file that is suitable for the purpose of dispaying in those places.

Thank you

I have problem of using the eraser function. Every time when I use it, I see the background color. Please advice. Also, please show me how to choose a medium size brush that is good for coloring the face. Thank you

What do you mean by 'the background colour'?
Did you use one of the eraser brush presets or did you put a brush preset into eraser mode using the icon on the toolbar?

In any situation, it's a very good idea to post a link to a full screenshot (.png image) that shows your Layers docker. Websites like Imgur and others can be used to upload and store images and then you can give a link to them here in your post.

If you use an eraser on a painted area then you would expect to see whatever is on the layer underneath, surely?
Without a screenshot of your situation, it's difficult to know what your problem is.

As for brushes, there are so many to choose from. I'd suggest the b) Basic-2 Opacity to start with. (Other people would give different ideas.) The opacity varies according to your pen pressure and you can change the size (and relative opacity) with the sliders on the toolbar at the top of the screen.

For background color, I mean the grey and white checkered square background. In other drawing apps, if I click on Eraser, I can erase the line or color I just did, the page will still be white or its original color (instead of white and grey checkered background). I use the eraser icon on the tool bar at the top of the screen.

Also, when I re-open the image I did, I can only use the mouse to draw instead of the graphic pen. Please advise.

What is the difference between "the eraser brush presets" and "put a brush preset into eraser mode". Please explain.

How to get the Basic-2 Opacity brush? Do I need to customize my brush? Thank you

The checkerboard pattern is a widely used convention which indicates that the image has transparency. It's what you see when you 'look through' a transparent part of the image.
You may be used to dealing with simple drawing/painting applications that don't have transparency. In these application, an eraser doesn't actually remove paint, it paints over it in the same colour as the defined background colour.
When krita starts with a new image, you have a white background layer (covered in white paint) and a transparent paint layer. It's intended that you paint on that transparent paint layer. You can put more paint layers (and other types of layers above that first one to have a multi-layer image.
You select which layer you paint on my clicking on it in the Layers docker.

The eraser icon on the toolbar sets the current brush preset into eraser mode. This means that instead of putting paint onto a layer, it takes paint off a layer, with the same brush characteristics. If you have a rough and textured brush, it turns it into a rough and textured eraser.
The brush presets docker, as well as having 'painting brushes', has three erasers, of different characteristics.
You can search for any brush preset by name (or part of their name) in the search box at the bottom of the docker. You select them for use by clicking on them in the docker.

All this is explained in the manual and in the many YouTube tutorial videos that have been made.

If your stylus (graphic pen) has stopped working, there could be various reasons for that.
What happens if you try to paint with the stylus?
One reason for this could be that a Windows update has broken your tablet driver (yes, this happens).
What happens if you make a New image and try to paint on that?

In order to be able to see all my saved file, I renamed them with A_. However, I now see them grey out on the screen but when I click on them, they cannot be opened. Please advise. Thanks

You really should make a separate topic for a each separate problem with a topic title that describes the problem. This will help other people who are having this problem to find the topic and perhaps be helped by any answers and discussion within the topic. That is the purpose of this forum.

The Recent Documents list is a list of the names of the documents that you've recently done work on. They are remembered by their names, e.g. picture1.kra, picture2.kra, etc. If you change the name to 'A_picture1.kra' the krita won't be able to find 'picture1.kra' because a file with that name does not exist anymore.

You can use File -> Open from the menu bar to open any file.

What should be a good size (width and length) in pixel is good for uploading the painting to the website? Can I use inches instead of pixels when creating the a new file? Can I use Krita to resize my painting before saving and how? Can I edit my painting saved in jpg format? Thank you.

Never, ever use jpg as your working file format. It's lossy. Always save all your work as .kra. Inches are irrelevant when you're talking about websites; pixels are. As for saving for the web, see: https://docs.krita.org/en/tutorials/sav ... e-web.html

@skrita: This topic has turned into a random mess of different subjects and isn't of much use to anyone, least of all you.
You really need to follow my advice about making a different topic for each problem that you have.

@ahabgreybeard: I have listened to your advice and the topic title I asked on April 8 is "Saving File for exporting" Please don't misunderstand.