Most sports shooters use 300mm or 400mm lenses.  You can get away with
a 70-200mm zoom if you are using a DSLR with an APS-C size sensor.

The best position by far is on the goal line between the goal and the
corner flag.  Halfway in between the post and flag, or slightly closer
to the post.  You can get good shots of both the player taking a
corner and the target man.

The best accessory by far is a monopod.

Traditionally, photographers would sit along the touchline between the
goal and the corner flag and you'll still find a lot of that today.
Some grounds don't allow anywhere else, so there isn't always a choice.

I personally prefer to sit along the side line, facing the goal about
10 yds up from the corner flag. With a 300mm lens on my D2Xs (450mm due
to 1.5x multiplier) this allows me to reach out in an arc towards the
centre circle and tight framing of the goal area - especially good for
corner kicks, making a much better angle than that obtained from the
touchline. This will be very tightly framed though, so I also have a
70-200mm on another camera for that use.

As for shutter speeds etc, I always work wide-open and the
corresponding shutter speed for whatever ISO you're using - F2.8 for
minimum depth of field as this gives much nicer 'muted' backgrounds.
This time of year, you're still OK with ISO 400 or if it's a bright day
you can go less though there isn't much difference on my camera between
200 and 400.

Good football photography is a skill that takes time to learn. You need
to learn to predict what's going to happen where and to be able to
follow the action often 'in advance' so you're ready for it.

Take yourself a monopod though as I do - holding a 300mm and a D2Xs for
90 minutes gets painful otherwise. When I'm changing camera to the one
with the 70-200 on, I can just swing it rapidly over my shoulder to one
side and pick up the other camera in one quick movement.

Good luck.