My orange is plain technical correction, but browny for style suggestions:

I will be a volunteer soon. I will have to help foreigners in Moscow. I think it will be very interesting. So I must exercise /practise. Please check the next /these /the following sentences: «You should go straight on /down /along the street, after the six story building turn right, and go straight till you see a /the brick building. That building is the museum you are looking for ["/which you are searching for", but "...museum you are looking for" is much more natural];

I recommend you to visit Old Arbat Street. [я замечаю, что кажется русские иногда находят немного неудобно английский использование "recommend."

I recommend you visit Old Arbat Street / I recommend visiting Old Arbat Street / I would recommend a visit to Old Arbat Street

Compare - I advise you to visit Old Arbat Street]

To get there you must walk down The Vozdvizhenka street from Alexandrovsky garden near The Kremlin. And when you see Arbatskaya subway station turn left and you will be on Old Arbat Street.; Please speak slowly, I can’t catch your words, I’m sorry.; Can you repeat (that) please.; Please wait a second I will call a person who can definitely help you.» <This is all perfect ))

I will have to help foreigners in Moscow.

I will be a volunteer soon. I will have to help foreigners in Moscow. I think it will be very interesting. So I must exercise (practice is a better word). Please check these next sentences: «You should go straight on the down this street, then, after you pass a six story building turn right, and go straight till untill you see brick building. That building is that museum what (or which?) you search (I don't understand what you mean by this). I recommend you to visit Old Arbat Street. To get there, you must walk down The Vozdvizhenka street from Alexandrovsky garden near The Kremlin. And when you see the Arbatskaya subway station, turn left and you will be on Old Arbat Street. Please speak slowly because I can’t catch your words have a difficult time understanding, I’m sorry.; Can you repeat that please.; Please wait a second, I will call a person who can definitely help you.»