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.