First there is the fun part of deciding where you are going to put in, but once you have chosen it, how do you get your kayak there? This is another elementary topic, but an important one to cover for new paddlers. If you have an inflatable or folding kayak, then this question is not an issue - the kayak travels inside your vehicle. But if you have a regular sea kayak, then it must travel on top of your vehicle.
Rules of the road require that your sea kayak is securely mounted on top of your vehicle with tie downs on the front and rear of your kayak. However, many mounting systems are secure without the front and rear tie downs. While I recommend them, and the rack literature will recommend them, you won't necessarily get pulled over if you haven't used them. If your load is unstable, then you will most likely get pulled over, and won't be allowed to proceed without fixing the problem (so have rope for tie downs with you just in case).
The kayak(s) must travel head-on into the wind - NOT at an angle. I've seen folks put their kayaks in the back of the pickup with the fronts leaning up against the cab. Then they tie this down. If you can imagine, the kayaks are facing into the wind at a 45 degree angle. This will put a lot of stress on the ties, the kayaks, and the vehicle. If you're going a very short distance and never exceed 25 MPH then this may be OK, but in no other situation should this be done.
How far are you going? If your put in is a short distance, and you don't have to use a freeway to get there, then you can get away with a short distance rig. I purchased foam cutouts that are flat on one side and concave on the other. I can place two of these on top of my '97 Honda Civic, and my Necky Looksha sits in them. Then I loop straps through the cabin, through my open doors, and I tighten these securely. This keeps the kayak very secure for short distances and low speeds (no faster than 30 MPH).
If you are going farther, you will need a rack. What kind of rack you need depends on what kind of vehicle you have and how much money you want to spend. Shop brands like Thule (my favorite) and Yakima. REI is a great place to start. Purchase two sturdy straps that have metal buckles with a thumb release. Bring along lengths of rope long enough to tie down the front and rear of all kayaks, in case you need to secure them.
Always check your rack installation before you load it up to make sure that all the fasteners are tight, and that the rack is secure. Bring with you any tools that you may need to tighten things up.
Kayaks should be mounted level on top of the rack, so that they point head-on into the wind. The bow or front of the kayak should point in the direction of travel. You can have the kayak either rightside-up or upside-down. This depends on your rack and your preference.
Once you have your kayaks on the rack, loop the straps around the kayaks and the rack in two places and tighten securely. Loop up the excess strap and tie this up with a slip knot. Flapping loose straps will cause annoying noises while you travel to and from your put in.
A final consideration is rack theft. Usually, it just takes a couple small tools to uninstall a rack, so consider getting one that locks into an unusable position. The Thule rack I use does this. Then when we are not kayaking, I uninstall the rack from the truck, so that while it is out in a parking lot, the rack is not a tempting target.