I need help working on my spikes and my overhand serve?

whenever i spike, i dont get that nice slanted hit to the ground, it sort of just bumps off the bottom of my palm and is like a floater serve really, How can i get this rapid, straight down spike. What exercises should i do or how long should i practice (also its pretty hard to practice because i dont own a net and without one its pretty hard to get into your game mode, if you know what mean.) Also my over hand serves, they go up (signifigantly high) and it doesnt seem professional. i want me overhand serves to seem like aced serves, almost all the time at least. I know that i must practice but what other exercises or things could i try out to improve both my spikes and overhand serves.