i can give you an example about question a) and b) then, i think, you will have some ideas.
i want to measure the average weight of my classmates.And there are 60 of them in all. Then the whole classmates is the population that i interest. And the weight is that measurement.
Thanks to sampling theory, i don't have to measure all 60 of them.Instead,i measure only some of them,for example,20 of them And then with this data,using some statistics method,i can get a conclusion of the average weight of them.
Sampling is the way to get some individuals from the population. In my i.e,i should use sampling to get the 20 classmates from my whole classmates.
random sampling is often implemented in this way:
1)label the individual of the population with a consecutive number.
2)get 20 numbers out of the whole randomly.you can do this just like lottery.
3)match the lots with my classmates which are the ones i should measure their weights.
i am not a English speaking one,so it's quite hard to tell things clear.
Hope i can help.