how will be the beijing weather in May first week.whom can I contact to get a guide.

The average temperature is 20C in Beijing in May, and there is little rain then. A short-sleeved T-shirt/shirt and long jean will be ok at day time. At night wear a light over cold additionally. You can just hire a guide through Travelchinaguide.
I think Wangfujing, Qianmen and Dashilan Street are good places to shop local goods in Beijing.