Just starting out, looking to downsample and antialias

Working on a toolset in Max MSP, and to maximize the amount that I can do at once, I’ve decided to shift my attention towards glsl for my 2D video processing. Just doing a (what I imagine is) simple gameboy camera emulator, for which I need to downsample and antialias. Where would I get start for this? I have a decent understanding how color is changed, I just got a simple luminance gate shader working, but I don’t really understand the process of downsampling or anything like that, or if I’m just barking up entirely the wrong tree. Any help would be appreciated!