$\text{1.}$ I started working as a volunteer 3 years ago.
$\text{→}$ started/ began + V-ing + time = have/ has + PP + for/ since
$\text{2.}$ She became a member of the Peace Corps 2 years ago.
$\text{3.}$ It has been 10 years since he last donated blood.
$\text{→}$ The last time + S + Ved/ V2 + was + time = It has been + time + since + S + last + Ved/ V2
$\text{4.}$ He has worked for this non-profit organisation for 5 years.
$\text{→}$ started/ began + to-V/ V-ing
$\text{5.}$ This is the first time he has such an interesting experience.
$\text{→}$ HTHT(-) = This is the first time + HTHT(+)
