Puredate (PD) is the free and open source version of Max/MSP, also developed and maintained by Miller Puckette. PD is one of the best options for people new to computer music, due to the obvious signal flow. It is a very helpful for exploring the basics of sound synthesis and programming but can also be used for advanced applications: https://puredata.info/community/member-downloads/patches In addition, PD offers simple and flexible means for creating control and GUI software.
The Sine Example
This first example creates a sine wave oscillator. Its frequency can be controlled with a slider: