Check out www.ThePerfectLoaf.com for all the info. You can make a starter following the directions there. If someone gives you a starter and you want to know the basics here is some info.
BuildASoil Sourdough Starter Feeding
Your starter will be very active and require twice daily feedings or you can put into the fridge to slow this down to once per week feedings.
How to know when I should feed?
VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while avoiding pest and environmental problems. VPD also controls plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress.
If you master VPD, you master your environment, and become better grower.