Contrary to what many Chinese people believe, it isn't always necessary to use a western name. Other nationalities don't change their names when they learn English or speak to people outside their own countries - English speakers can cope perfectly well with Russian, Turkish and Arabic names, so there's no reason why we can't cope with Chinese names, too. In your case, though, it might be wise to change your name slightly. Westerners may presume your name is pronounced the same as 'See you' (meaning 'Goodbye') , which could cause confusion, or could even result in people teasing you. Go for something as similar as possible to your own name. My name's Susan, but I wouldn't recommend it because it's a little old-fashioned. How about Sue or Susie, perhaps? Susie Yang sounds nice. Or Sienna?

"Izzie" is a common nickname for "Isabel." It sounds similar to your initials, "YSY," pronounced as one word.

I think a good way to get an English name that is decent is to do a search for "most popular boy/girl names for 2017" and choose one of those. If you want a name that sounds a little like Si Yu you might want to to choose Sue, or Sia. Cheers!