where should we use the formulas of sum of n terms of ap and nth term of ap
It completely depends on the situation you are given with.
For example, consider that you save Rs 200 in first month and each month you increase your saving by Rs 25. Let say you want to find the saving in 12th month.Then you can use the concept of nth term of an A.P. to find it. If you want to calculate the total saving in 12 months, then you need to calculate it by using the concept of sum of n terms of an A.P.
If you want to find only one term of the A.P. , then you need to use the concept of nth term of an A.P and if you need to find the sum of some certain terms of the A.P., the you need to use the concept of n terms if the A.P.