If you're new to screencasting and want a simple tool to get started we recommend trying Kaltura Capture first. If you'd like a program with more features or are having issues with Kaltura Capture, OBS is a much more powerful program but takes a few minutes to configure when you first use it. This video shows you how to quickly configure OBS to record screencasts.
There are many OBS tutorials available on YouTube, so if you'd like additional information on using OBS you might look there.