Terrific advice in this thread, definitely start on flat ground (hell i started off practicing ollies in the grass) and wear helmet/pads (some people might give you shit about these; those people are idiots), BUT:

You absolutely will eat shit from time to time, and you have to accept this and be ready for it. I must have tumbled down the side of the bowl 50 times before I finally dropped in successfully, and it was worth every scrape and bruise.

Someone else said "learn to fall." I'll go one further and say if you have the means to do so then take a judo or parkour class - they will teach you how to minimize injury when you hit the ground.

Also, it's not a terrible idea to walk an area before you skate it, and note large cracks/gaps in the sidewalk, scattered bits of gravel, etc. so it isn't a suprise when/if you run over them.