Перевидите что б понятно было

I have not decided what I shall do when I become an adult. Choice of vocation is not a simple matter, because the interesting professions in the world very much. Perhaps I'll be the doctor and will be able to overcome the insidious disease. Or, piercing the space, visit other worlds. Maybe I'll explore the mysterious corners of our planet, or engaged in the cultivation of bread. And maybe I'll go to the scientists and find a new energy source, or just invent a kitchen robot that will instead of my mother to cook the lunch and wash the dishes. I'm only in fifth grade, and it's hard to say who I am. But I perfectly understand that the knowledge gained in school will help me in the future, when I get to work for the benefit of society.

Do you know where the Mississippi River is?
THE is the definite article.   when you have ONE of something that is definite, you use THE.  there is only one mississippi river, so you must use 'the'.

'an' is an indefinite article, not appropriate. if you have one of something thats not specific such as 'an apple'. 

He will be twenty in a month.
'in a month' says this is in the future, so you need a future tense verb
'am' goes with 'I' always  (I am ...)
'is' is present tense, but you could say 'he is going to be twenty in a month'

We might go to Peru next year.  We're not sure.
'not sure' means it may not happen. 'might' is also uncertain
so 'should' means we need to do something

that's the woman whose son is an actor.
i am not sure why, just that this one is correct

This afternoon, we are going sandboarding
a) and b) are the same answer
'this afternoon' means future tense
'going to' states a plan or intention, where 'will' is used in a different sense, as in a promise or request for help

i'm bored of camping trips.  let's go on a safari
'for' is definitely wrong
i have heard 'from' used, but i suspect it is not proper grammar

извините за все английский, мой руский не достаточно хорошо.