Basal Body Temperature (BBT) should be taken with a right thermometer: temperature curves cannot precisely be read if taken with the wrong thermometer. Choosing the right tool for you is the first step.
You can find a thermometer to start charting at the beginning of the course.
There is no need for a fancy and expensive tool: you can find good ones in a range of 10 to 20€ (and my guess is that the price range should be the same in USD). However if you have an advanced thermometer/computerised tool that you previously used to chart, you can continue with it.
The most recommended is analogical : a glass thermometer (the old ones with mercury, the new ones with gallistan). Some thermometers have a specific large graduation between 35 and 39°C to check BBT.
Otherwise you can choose a digital thermometer with two decimals. This option can be best if you tend to break things (after 6 broken glass thermometers I decided for a digital plastic one!), but you should be careful about low battery: it can disturb good BBT readings.