Remember that repetition is not your enemy.

Repetition is nice in a musical phrase.  You should repeat musical phrases often but not too often.  Use your judgement.  Long musical phrases are nice, but they need to be repeated in at least 2 strophes.  Preferably more.

C major and A minor are good keys to start in.

Especially for beginners, but they can be classic songs for sure in those keys.  Try starting with an elision from the circle of 5th, like

I-->ii-->V-->I 

and then, let's add a different pattern afterward.

I-->ii-->V-->I-->vi-->IV-->V-->I

Then repeat the whole pattern twice.

I-->ii-->V-->I-->vi-->IV-->V-->I-->I-->ii-->V-->I-->vi-->IV-->V-->I

Then push to another key called a modulation.

I-->ii-->V-->I-->vi-->IV-->V-->I-->I-->ii-->V-->I-->vi-->IV-->V-->I-->I-->ii-->III-->vi

Now tail it off a little.

(vi)-->VI-->ii-->V-->I

Now you've built a musical phrase.

Chords, the lifeblood of song.

You must learn the circle of fifths in order to become a good musician.  You need to move from and to different keys on the keyboard or fretboard.  It is important to practice different hand positions on any instrument.  You have to be careful about which chords you use.  Here's the basic pattern for the circle of 5ths.

1-->3-->6-->2-->5-->1

You can also move in thirds as is done in the first chord change of this passage.

As you break it down into major and minor keys. Remember that the majors and minors of each hand position are different major and minors.  Let me demonstrate by using capitals for major hand positions and lower case for minor hand positions.  These are the two hand positions.

I-->iii-->vi-->ii-->V-->I  for major keys

and 

i-->III-->VI--> flatII-->V-->i

This is the circle of fifths.  Know this and internalize this.  It will improve your music.  You can also use elisions to the 4, and you can use any individual portions.  Your music should be as varied and interesting as you can make it.

Songwriting: A fervent art.

You must always hold songwriting as a fervent are.  The Beatles weren't just big for the way they sang, they were practiced and excellent songwriters.  That's what made them famous.

But what makes a good songwriter.  I would have to say depth of musical thought, as well as depth of lyric.  You need to mould the two together.  Instrumentals can be fun and insightful musically, but there is nothing like a crafted lyric with a song to set your mind at ease, awaken you to new things, and summon your soul to you.  The more you practice this, the better you know who you are.  But never hate on yourself, this only leads to trouble.  You must praise yourself for the good things about your music and not let the self-criticism lead you to stop playing.  There will be plenty of critics in the world.  You should try to correct yourself only when the music is sliding off.  You don't want a trainwreck in your music.  Keep playing even if you mess up.  Don't stop and fix things while you're playing.  Wait until you finish to give yourself and others criticism of how to do your song.

Being a songwriter is a lot like being a teacher.  You are always teaching new parts to people in an effort to create the best possible sound for each song.  You must remember that with teaching comes learning.  You learn best when you teach and this karma of self criticism will lead others to be critical of you as well and this is not always a bad thing.  They'll say they like something but they don't like something else and this is a healthy way to be.  This relationship should never become dysfunctional however.  And there are a lot of negative influences in the music business.  Try not to succumb to them too much.  Lord knows I did and it negatively influenced my life.  Try not to get too attached to girls too young.  Or boys either for those women songwriters out there.  You need to pursue your career as a songwriter.  You must perfect your talents and put on the best possible shows with what you can do.  I believe in you.  Believe in yourself.