how a friend can inivite me to china? how he/she can send me invitation latter?

You can apply for tourist L with an invitation letter from your friend. The letter should include following details:
1. Information on the applicant (full name, gender, date of birth, etc.)
2. Information on the planned visit (arrival and departure dates, place(s) to be visited, etc.)
3. Information on the inviting entity or individual (name, contact telephone number, address, official stamp, signature of the legal representative or the inviting individual)