As English is my sencond language, I have mother tounge influence. I have problem with pronunciation of letters 's' and 'sh'. Also I have found Phonemic chart to improve my pronunciation. but now i am a little confuse that which set of words should I choose for practise.

Start with this classic:

She sells seashells by the sea shore.

or the riskier:

I slit a sheet, a sheet I slit. Of all the sheets I've ever slit, this sheet is the slittiest.
Thanks Mister Micawber. Your post is really helpful. Would you help me to find some more such sentences?
Students: Are you brave enough to let our tutors analyse your pronunciation?
I don't know an on-line source. Here are some from my workbook:

I know Shick's been sick for a long time now.

With the strike on, not a single shingle moved.

She sings in a Flushing singles bar.

My sister's shishkebab is delicious.

Sylvia's furnishings are refreshingly simple.

One of Shipman's sisters is in Chicago, and the other is in San Francisco.

The Cushing syndicate owns sixty ships.

I bought my silk shift in the Chiffon City shop..
Thanks Mister Micawber!!
Well, you seems to be from India. You can pronounce both sibilant s and shibilant sh easily. (You need to work on voiced sibilant and shibilant--as they are not present in many Indian languages) Your problem lies elsewhere: which grapheme corresponds to which phoneme? Just look for English phonics lessons on the net to solve that problem.
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I have also been suffering from the same whistling s and can't differentiate between s & sh sound. And unfortunately I had been rejected in one interview of teacher's job for this speech problem.please help me by giving the direction of tongue and teeth positions while pronouncing.being an English teacher its my backdrop