Dill: Harvesting and Use

I was given a dill plant and I put it outside. It has grown quite tall and I want to know when do I pick the dill off and dry it to use. Some of it has gone to seed and I want to know if I can use any of the part that went to seed. This is my first experience with it and I'm hoping I'll get some good use out of it. Also does the plant come back each year or do I have to buy a new plant.

There are two uses of dill: the fresh leaves or the more or less ripe seeds. The leaves can be picked anytime, but the younger they are the less tough they will be. Just make sure not to strip the plant since it needs some leaves to make food for itself.
The leaves are used chopped in salads or mixed with sour cream and other spices as a sauce or a dip.
The seeds are used to flavour dill pickles. The seed heads are stuffed into the jars with the cucumbers and the rest of the spices and hot vinegar and water is poured over it before it is pressure cooked or cooked in a hot water bath. I find as long as you don't mind softer pickles, you can dispense with the prior brine bath for the pickles and this way get a less salty pickle to boot.
Dill is an annual, so your plant will not overwinter. You can save some of the seed for next year, but if you want faster results a new plant is a better idea. Sow seed where it is to grow,(a sunny spot with fertlile well-drained soil), since transplanting tends to set it back badly.