How can I write a good composition without grammar mistakes ?Also, while writing a composition, I feel that either I have no ideas or too many...

The easiest way to avoid sentence level
grammar mistakes when writing compositions is to memorize the simple Who
What model of English sentence structure. This model is the same as the Somebody
Something model you may have seen utilized in dictionaries, such as href="http://www.ldoceonline.com/dictionary/throw_1">Longman Dictionary of
Contemporary English, as in the definition of
throw:

readability="8">

throw something to somebody: He threw his shirt
to someone in the crowd.
throw something at somebody/something: Someone threw
a stone at the car.
throw somebody something: Throw me that towel, would
you.

These models can guide
you to constructing sentences that are free of error. The Who
What model asks these questions: Who Did What to Whom Where When How and
Why. It's rather like the prescribed accusation in the world famous
Clue game: "I suspect Colonel Mustard killed him in the Ballroom
with the candle stick [for the purpose of getting the jewels from the
safe!]."

Look at that Clue sentence
this way: "I suspect Colonel Mustard [Who] killed
[Did What] him [Whom] in the
Ballroom [Where; we might insert "yesterday" for
When] with the candle stick [How] for the jewels
[Why].

This model provides the
basic order of an English sentence. Of course the elements
of How, Why, When and Where, as adverbials, may be moved to
other locations in the sentence [e.g., I suspect that yesterday he was killed for the
jewels with the candle stick in the ballroom by Colonel Mustard, etc] but until you are
very confident with the Who What model in its standard form, it is recommended that you
use only the standard form.

Precisely the same thing can be
accomplished through using the Somebody Something model, which uses assertions instead
of questions. The only advantage of one model over the
other relates to your preference: If you prefer to think in
questions (Who? Did what? etc), then use the Who What model. If you prefer to think in
assertions (Somebody Did Something etc) then use the Somebody Something model. The
Somebody Something model goes like this: Somebody Did
Something to Somebody Somewhere at Some Time with Something (or Somehow) for Some
Purpose (or for Some Reason).

Word level
grammar has many more points to it, but some things that commonly affect
writing are href="http://owl.english.purdue.edu/owl/resource/599/01/">noun - verb
agreement, articles, and
prepositions. Firstly, nouns and verbs must agree
in number. This means that when you have a plural noun, you must also have a
plural verb. A singular noun (one horse, deer, plate, moose etc.) goes with a singular
verb (e.g., is, has, runs, walks, goes, falls, etc). A plural noun (two or more horses,
deer, socks etc.) goes with a plural verb (e.g., run, have, are, walk, go,
etc.).

Secondly, especially in American English, nouns most
often require an article (a, an, the) in front of them unless they
have a noun or pronoun in front (our car; Martin's car): a decibel, a door, the door,
the car, the hospital, a hospital, etc.

Thirdly,
prepositions can be tricky to use as some overlap in meaning. For
preposition use, the best advice is to look up the meanings and common usage of the
prepositions in question in a dictionary; you'll find guidance from a dictionary about
which preposition to use (e.g., I might have written: you'll find guidance
in a dictionary on which to
use).