As long as you signed up to have the ability to trade options, you can write covered calls since all you need is the most basic option permission, level 1.
a) Each option represents 100 shares of the underlying stock. So the minimum number of contracts you could write for a covered call would be 1, which would mean that you must be long (own) 100 shares of the underlying stock. So for every 100 shares of the stock you own, you can write 1 covered call.
b) Basically, if you already own long positions then you can write a call on those positions, at a ratio of 1 contract to each 100 shares of your long position.
Take this for an example,
If you are already long 200 shares of AAPL purchased at $180,
You can write or 'sell-to-open' a maximum of 2 call contracts, for example at a strike price of $190 with a $2.22 premium.
As long as your shares of AAPL do not exceed the strike price of $190 by the options expiration date, you will receive a (2.22*100) $222 premium for each contract, or $444 total, in addition to your gains/losses on your long position.
Covered Calls will limit your upside potential though,
If AAPL does exceed $190, say it went to $210. Then when the options you sold get exercised, you will have to sell all 200 shares (100 for each contract) for a price of $190. You would receive your profit of $10 a share for each share of AAPL and still receive you're premium of $444. (10*200) $2000+$444=
$2,444 profit. But if you had just held onto the long position, you would had profited (20*200)=$4,000
Just a couple of notes though, I didn't factor in early exercising of the options (which rarely happens anyway) or fees (which make selling far out of the money options pretty useless)
Also note that if you do not have the ability to write naked options on your account, you will not be able to sell your underlying long position until the options you write expire or you 'buy-to-close' the same number of call contracts that you had written with the same strike price
I use covered calls relatively frequently to get a little extra cash each month, they are especially useful for long positions that are not very volatile.
Hope this helps,
-mgrass