What should I sidechain to the kick?
Set the threshold low enough so that you get around 6 dB of compression on the bass every time the kick hits. You should be able to hear the compression, or at least you should hear the kick pop out a bit more than it did before. Now you can tweak the compressor settings to your taste.
How do you do a sidechain kick?
To sidechain your kick & 808 load a compressor on the 808's channel and send the kick's channel to that compressor. Then, make sure that the makeup gain is 0, lower the threshold until you hear enough reduction, set the attack to (0ms-10ms) and the release to (40ms-150ms).