About difficult Russian letters

When we pronounce letters "Ж", "Ч", "Ш", "Щ", their sounds aren’t in Greek language. Therefore it difficult pronounces correctly and clear.
But you have to remember in writing Russian we always use "ЖИ", "ШИ" (never "ЖЫ", "ШЫ"), "ЧА", "ЩA" (never "ЧЯ", "ЩЯ"), "ЧУ", "ЩУ" (never "ЧЮ", "ЩЮ"). In speaking say "ЖЫ", "ШЫ", "ЧЯ", "ЩЯ", "ЧЮ", "ЩЮ" (any way you can’t say it another).