How do I hook up a sprinkler timer if the power wires are yellow and red and my timer wires are black and white ?

The easiest way to handle this is to search the vendors site for installation instructions,

More than likely, Red will connect to the Black Hot lead and Yellow to Neutral or White.

Also check the timer itself to see if connection instructions are already on the back.

I agree with donf the internet will usually be helpful -- someone knows the answer

Many manufacturers have web sites with tons of information -- Google Mfgr.

What Timer do you have? Does it control 24 volts out?