The yield on shares is a bit like the interest rate on a savings account in the way it is calculated. So it is dividend divided by the current price. So if the annual dividend is 25p per share and you pay 167p per share the yield would be 25/167 which is 14.9%. However, as the price goes up the yield goes down which is why tipsters are currently saying that shares with low prices but high dividends are good buys. Taking some of your other questions:
I'm not sure what previous yields have been for BT but as the price would have been higher yields would have been lower in the past. You should google BT dividends to determine what the dividends have been for the last few years.
Nobody decides what yields will be - it is a function of the profitability of the company (which should influence dividends) and the market which determines the price. Dividends are variable and are decided by the management of the company.
Dividends are allocated to all shareholders on prescribed dates of the year and are generally paid six monthly. There is a date which is called the ex dividend date which determines whether you will get it or not - so long as you held the shares on that date you would get the dividend payment even though you may have sold them subsequently. The BT website should be able to give you the relevant dates.
The price will vary around the ex dividend date as obviously one day you are entitled to a payment but the next you are not and the market will factor that in.
As you can see buying shares isn't an easy process if you aren't experienced. You also need to make sure you've got a broad spread of assets and are sufficiently diversified ie. not all your eggs are in one basket. You should consider your existing portfolio (cash, ISAs, pension, life insurance etc) before considering venturing into shares.
A decent IFA should be able to help you and make sure you understand what you are getting into. Having said all that over the longer term (at least 15 years) shares should out perform cash but be prepared for a roller coaster ride !