The Electric Reliability Council of Texas (ERCOT) does it for you. When you sign up for a plan with a new provider, ERCOT will send you a mailer confirming the switch. You have three days upon receiving the mailer to change your mind. If you don't, you'll have a new provider within seven days, and ERCOT will notify your old provider. Just remember, if you abandon a contract before it's complete, you will be on the hook for any fees or penalties detailed in its Terms of Service.
Price, of course. Prices are expressed in cents/kilowatt hour. Plan types include fixed rates and variable rates. Fixed rates offer consistency throughout the plan term, which can run from six to 36 months. Variable rates can change monthly - they're great when prices go down, but not-so-great when they don't. Finally, if you care about your carbon footprint, you may choose a plan sourced by solar or wind energy.