Linguists call this pronunciation S-retraction or S-backing, because the SH sound is made with the tongue slightly farther back inside the mouth than it is for the S sound. So you're moving the S farther back in your mouth: S-backing. There is no one definitive answer to this question. Some people pronounce their s as sh, while others pronounce it as s. Some people might say that they pronounce it the way their parents or other relatives do, while others might say that they pronounce it the way they hear others pronounce it. Ultimately, the way that you pronounce your s is up to you.