What is the best way to cut a sink hole in a kitchen countertop?

I have never done it before and i am afraid of messing it up

Measure, then measure again and at least 3 times...the circumfrance of the sink.....MINUS the distance from the underside of the sink to the outside of the underside.
Cut a hole in the very middle first........remember you can always make the hole smaller but never bigger.
Depending on what it is made of you will need to go with whatever TOOL will cut the material of the countertop.
Probably start with a Reciprocating saw.

If you are really scared practice on a piece of scrapwood till you get the hang of it.

Also go to the DIY network and see if you can watch a video of it being done.

it is scary, but really quite simple. Just make the outline of the hole is smaller than the outside diameter of the sink before you cut it. You don't want it so tight that you can't move the sink around after placing it in the hole. Use a skill (circular) saw for the straight cuts, and a jig saw for the corners, or cut it all with a jig saw.

Use a jigsaw with a down stroke cutting blade ,And a circular saw for the straight bits,Place sink upside down on worktop draw round it,Then draw a line 15mm inside it,Cut to the inside line.

Put a few layers of wide masking tape around the area where you are going to make the hole. Turn the sink upside down and place it where you want it then trace around the edge. Move the sink out of the way and look at the tracing, measure it and make sure that it is square and parallel. Now using the template that came with the sink draw the line where you will cut on the tape, drill a hole through the countertop and using a saber-saw cut the large sink hole out. Make sure that there is no plumbing or electrical in the way underneath the counter. The hole should be at leat 1/2" smaller than the sink so that the sink rests on the lip and doesn't just fall through the hole. The tape will protect the countertop from scratches and nicks and give you a surface to draw on. Measure twice and cut once, you won't get a second chance. You can do this, your not stupid you just never did this before. Take your time and everything will be fine. Oh, read any directions that came with the new sink first.

Mark it out,drill a hole in one corner,then use a jigsaw

to cut it out. Just make sure you do not mark the outside edge

of the sink unit

Measure very carefully the hole you will need... don't use the lip of the sink or it will fall right through. Make sure you've got your spacing planned out. Then drill a hole and use a jigsaw from that hole. Remember that you can always make it bigger but not smaller. Be conservative with your measurements the first time you cut.
ALSO many sinks come with a cutting guide that you just trace on. Maybe yours does too.

The box that the sink comes in usually has a template marked on it. Turn the counter over if it is not attached already. Cut out the template--usually "dash lines" in a sink type pattern--and lay it over the top of the counter where you want the sink to go. Make sure you check underneath the counter to assure you have no obstructions or problems in connections for drainage.
Mark the outline of the template onto the counter, whether or not it is already connected. Give room for the backsplash, usually the box will give instructions. If no box, go to a hardware store and read the instructions that are on a box with a sink in it that is the same size as yours.
Kitchen sinks have a lip all around them, giving you room to make a cut smaller by about 1/2" than the basin portion and still hold the sink when you set it to the countertop.
Drill a hole to accommodate a jig saw blade, near the edge but inside the marked line.
Here is the tough part. I use a B&D jig saw with a fine blade to make the cuts. However, all the blade teeth are aligned for the cut to come upwards. Therefore, if the counter is already set on the cabinets, the teeth will cut upwards potentially chipping the formica or surface of the counter. I take good masking tape, usually two inches wide, and lay it all around the top of the counter where the template will go, and then mark the cut lines on the tape). Carefully start making the cut, moving slowly along it.
You will have problems with the corners, even though they are curved a bit. Keep using the jig saw blade to round off those corners. you don't have to be precise, but you also don't want too much taken off the corners or the lip of the sink won't cover them.
Then, use clips you can buy at a hardware store to hold the sink down after you set it in place. Usually six clips will hold a sink in place. You can (I don't) "set" the sink by running PL200 or some other adhesive around the outer edge of the cut for the lip to sit on. Buy some waterproof caulk and press it into the joint of the sink lip and the counter, wiping off the excess with a damp cloth.
All of this should be on instructions on sink boxes when you buy the sink. Barring a box, use above but do study an existing box instructions at the hardware store.
Connect your faucet etc. through the preset holes and make your water connections.
It really isn't as hard as it sounds. But the first time is usually the hardest. By the way the clips for a sink include a small screw that holds a rectangular piece of metal against the bottom of the counter. Ask when you buy them. They are cheap.