For some reason my former employer did not withhold any taxes from my paycheck last year. how can I avoid this in the future? did I fill out my w-4 incorrectly?

After submitting your W-4 to your new employer, check your next payroll stub to see if things look reasonable.

If not, take corrective action immediately 

It sounds like you're right, the w4 might need to be changed. Here's a calculator that can help guide you on how to change it: https://turbotax.intuit.com/tax-tools/calculators/w4/