Once you have found a Witch Coven (if you don't know how, click here), you simply walk up to the figure that keeps changing from a young woman into an old hag and click on her.  This brings up a menu.  Selecting "Summon Daedra" will cost you a couple of hundred thousand gold, but you might get lucky. Selecting "Quest" will (hopefully) generate a quest for you.  Your rep with the Coven is very important because it not only reduces the cost for Daedra Summoning, but it also increases the chances that you will be able to get a Quest without having to keep clicking until you get lucky.

The only oddball thing about Coven quests is when you finish, you don't click on "Quest" as you would for the Guilds.  Instead you must click "Talk" as you would for the Merchants and Innkeepers.

I think the Coven quests are some of the neatest in the game.  About half of them will net you a magic item of some sort as your payoff.  

"We would use thee to find a a perfidious document circulated by our enemies..." (any rep): travel to the temple and get the item (you'll usually have to fight a couple of guardians) and return it to the Coven. You get a magic item for your reward.

"The <coven> actually could use thy assistance in a most absurd and yet serious matter..." (any rep): take the child to its home and come back for your reward (a gem). You'll get to fight a few fanatics along the way.

"<Coven> have been contacted by the Mages Guild and asked about a special potion, a very sacred formula much desired in <province>..." (any rep): this is the "universal cure", so you'll get to fight vampires and lycanthropes while you are trying to deliver it. "Use" the item to cure your own disease, but you'll still have to fight them (and run the risk of reinfection). This will ruin the item and blow the quest, by the way, so make sure you have a little rep with the Coven under your belt before you blow this one.

"We have a potential problem I fear. A vampire ancient hateful of witches hath moved into an abandoned castle in our territory..." (any rep): a standard dungeon crawl with a Vampire Ancient as your object. Go prepared for a fight.

"Our sister has been in imprisoned. We need someone like thee to rescue her..." (any rep): a standard dungeon crawl.

"Yes, as a matter of fact, thou could help us.... We are an isolated coven, viewed with fear and hostility..." (any rep): pick up the book, avoid the guards, get a magic item for your reward.

"Thou wouldst help <coven>? Many come to us for aid, and then turn from us..." (rep between 0 and 9): deliver the bracelet to the contact (who will change into some sort of critter and attack you) and return to the coven for your magical reward.

"If thou wisheth the help of the <coven>, first thou must help us. Will thou bring us the three ingredients we need to complete our brew..." (rep 10 or more): retrieve 3 items from 3 different dungeons. Other than that, it's standard dungeon crawling with a decent reward at the end.

"Thou art friend to <coven>, this we know. But to help thee, a spell must be woven..." (rep 20 or more): retrieving the item will cost you 1000 gold. Pretty standard, otherwise.

"...We require a heart of a daedra for our spellcraft but the heart must be recently plucked and by thou..." (rep between 40 and 49): a standard dungeon crawl with Daedra as your target (any Daedra will apparently do).