under the viewer there is the icon activating keyframes. go to frame 1 do your shape, go to frame 100 modify it, the key is done. You cannot move a key in the roto like in after effect, you have to created it or copy it (right click...)
for your seed, there is a similar key icon next to the name to set keyframes on in the parameter. You can move theses ones in the curve editor.
You can also put an expression in your seed intead of of a value, it will make more sens. try something like :
modify that to get what looks fine.
Thats, really really basic questions, you should do some tutorials to learn shake a bit before