If you haven’t done so already, you should download Reaper. It’s a different kind of DAW. Reaper is highly customizable from many standpoints and one of the most noteworthy is the ability to code your own plugins from standard text docs using their JSFX language. This makes learning DSP a breeze =D
Get started today…
A minimalistic simple gain example:

desc: my_plugin_name
// comments start with double slashes
slider1: 1<0,1,0.01>linear_gain
@sample
spl0 *= slider1;
spl1 *= slider1

… in this case, slider1:
– starts out equal to it’s default value, 1
– can be as small as 0
– can be as big as 1
– and has 100 steps (step size = .01)

Therefore, each output sample will be less than or equal to the input. Essentially, the slider will set the output amplitude to 0%, 1%, 2%, …, 100% of the input amplitude.

Have fun!