Until you are an experienced craftsman, introduce the important characters as early as possible. The reader should know them as long as possible if he is to take a keen personal interest in them.
It is better not to describe your characters more than is necessary for actual identification; they should describe themselves by their actions and conversation, as the story proceeds.
To save the monotony of long descriptive passages, that always hamper the movement of a story, it is often possible to make one of the characters, in the course of conversation, give the information that the author is anxious to convey to the reader. But in order to effect this, do not fall into the error of making a character say things that in real life there would be no reason for his saying. You may want to convey the information to the reader that the heroine's ancestors were eminently respectable; but it would be bad art to make her remark to her own parent (or a relative): "As you know, mother dear, grandfather was a distinguished general."
Beginners imagine that the strength of a story is in direct proportion to the way they crowd together incidents, or multiply their characters. But this entirely depends on the quality of the incidents and the importance of the characters.
The whole is greater than a part—always has been and always will be; and if each individual character is weak, and each episode is feeble, no matter how you may elaborate your story, the whole will be weaker than each part.
It is time-saving, when writing a story, to lay the scene in some locality you know well, even though you change the name and preserve its incognito. It is most useful to have a fixed plan of the streets and lanes and buildings and railway station in your mind when writing.
Try to distinguish between a longing to voice your own pent-up emotions, and a desire to give the world something that you think will interest or instruct them. Three-quarters of the love-stories girls write are merely outlets for their own emotions; and picture what they wish would happen in their own lives—with no thought whatever as to whether the MS. contains anything likely to interest the outsider.