Gonna answer your questions in reverse order.
3:how to import characthers
Either put the sprites in your project's folder directly (YourProjectName\Graphics\Characters), or go to the top of the editor and click this button:

Then just select the characters folder on the left and hit the 'Import...' button. If the image isn't transparent, it will ask you to select a colour on the image to make transparent (so just choose the background colour of the image).
2:what size when am making a sprite sheet should it be
Character/Sprite sheets, assuming you're using no scripts to change it, are able to have either one character, or a set of 8. Cynwale and Andar linked some good resources above, so I'll just say here that
- For sprite sheets with only one character, you have to put a $ at the beginning of the file name or it won't display right
- Most default RTP sprites fit on a 32x32 pixel grid
- Individual character sprites are comprised of 3 columns and 4 rows of equally sized squares. (in RTP sprites, the squares are usually 32x32 as stated). You can't have the middle frame be smaller than the left or right frame, all columns/rows have to fit on a perfect square grid.
- Group character sprites work the same as the above, they're just all on one image, with two rows of 4 characters.
1:when making sprite sheet does should i make a idle or no because as i check rpg maker sprites
The characters' middle column on the sprite sheet are the idle/standing sprites. If you want a separate idle animation that's not also used in the walk cycle, though, there are scripts available if you look for them.