As you might know, Russian consonants sounds are either hard or soft. Ш is a hard consonant, Щ is its soft counterpart. I recorded an audio file with 6 pairs of words where Ш and Щ are in the same position, so you can listen to it and compare: https://audioboom.com/boos/1890544-distinguishing-and

Then try to say =) ш - sh щ- sch

بص يا صديقي Щ امثلة على شمس щамс شبشب щибщиб Ш امثالة على شكرا шукран شوربة шурба

Щ is longer and has most of your tongue pretty high. Ш is a normal length consonant, and you tongue is rather low, and a bit bent back, like a spoon. Listen to "шуба" and "щука" at www.forvo.com. I recorded these two words the same day to show the difference.

BTW, When I say Щ, sides of my tongue touch my back upper teeth:/ I don't know if it can help...