Coming to Beijing requires a visa. There are different categories of long term visas depending on your specific situation.

Z visa - Work: given to foreigners working permanently in China and their families.

X visa - Student: given to students and interns staying in china for 6 months or more.

F visa - Business/Short term study: given to students, professionals and businessman, staying in China for 6 months or less.

D visa - Resident: given foreigners permanently located in China (holding resident's certificate).

For further information you can consult this website:

http://www.china.org.cn/english/LivinginChina/180480.htm

In Beijing you can also go to:
Division of Aliens and Exit-Entry Administration of the Beijing Municipal PSBNo. 2 Andingmen Dong Dajie, Dongcheng District
Beijing, 100007
Tel.: 8401-5292-5293, 6404-7799 ext. 4301/4302
北京东城区安定门东大街道２号　　邮编：100007

If you want to go to China and try to find a job there, the easiest way is to arrive on a tourist visa. Then if you manage to find a job, your company can obtain the nesesary document to help you get a working visa (Z or F).

As these visas are not very easy to obtain, be sure that your company agrees to do this for you before signing your contract.

Some agencies also offer a visa service. For a commission, they can get a F visa (up to 6 months) for you. As with so many other things in China, it is not really legal, and is mostly ignored by the authorities. So far, it is tolerated.