You can use a regular thermometer, but it won't give results as accurate or specific as a basal body temperature(BBT) thermometer.
BBT thermometers have a number of advantages: They're speedy, durable, and accurate to a 10th of a degree. Plus, they can store your temperature if you don't want to chart it right away.
Whether you choose to use your trusty old thermometer or splurge for a BBT thermometer, there are a few tips you need to keep in mind to ensure that your readings are accurate. Always take your temperature first thing in the morning, before you even think about getting out of bed. Also try to take it at the same time every day for the sake of consistency (and make sure you get enough sleep beforehand!).
Once you've seen what a month likes look on your chart, you'll be armed with the tools you need for TTC success. Now get busy!