>>1-What's the first value of the previous output value?
Of course zero. Imagine you begin process from silence. and after for ex 5 sec of silence you obtain real signal. Zero begin state is commonly used init state
>>2-How to decide sampling time or α coefficient in the example?
You can regard impulse response of this filter (reaction to 1,0,0,0,0,.... sequence)
it will be
1,alpha, alpha^2, alpha^3, and so on
Considering sampling period, you can estimate how long time period covers your filter