how do you compose a song in c major scale?

It depends on if you mean composing in the classical sense or the modern sense. If you mean the classical sense, than I would start with a simple melody, and then add the appropriate harmonization with chords. If you mean the modern sense, then I would mess around with your instrument and play these chords until you find the right progression for the sound you are looking for; C major, d minor, e minor, F major, G major, a minor, b minor. I personally like the progression C major- G major- d minor- a minor. After you get the progression of chords you like, start writing lyrics and singing them so that they fit into the chord progressions. That is how I go about it, even if I already have the lyrics written down. The first person to comment is right though, composing is a lot more complex than just doing it. If you need more help, feel free to email me. tcarmona0@rocketmail.com

I will do the best I can to teach the essentials. Oh, and the last thing you want to do is force a song out... it just gets messy. You have to let it flow.

That is a much bigger and deeper question than you realize! But, you can start by writing your lyrics down. Then if you play an instrument start chording. Next, start singing or humming with you lyrics in mind while following a simple chord progression in this case something like C F C G7 C. You will want to do several measures not just one. You wont have to worry about sharps and flats as this is C major and does not have any.

Finally, you may wish to (or not) substitute the C in one or two measures with Am. This is a simple and effective way to begin writing songs. Years can be spent in song writing as the complexities can be enormous. Good luck!

A minor has G#. The key signature is the same but the accidental G# is added. there are several types of minor scales. One raises the 7th note a half step above the key signature and one raises it only in ascending passages and returns it to key signature position in descending passages. I am sure you can find a full explanation of minor scales on the web.