Where am I wrong?

Make one relative sentence from two. Use the sentence in brackets to make a relative clause. Join them with who, which, that and where

Example:
-Cancun (I have a friend there) it is a beautiful city.
-Cancun, where I have a friend, it is a beautiful city.

1. A bullfighter is a person (He kills bulls in a bullring)
A bullfighter is a person *who kills bulls in a bullring. who-> person; that-> object 

2. My best friend (He is a taxi driver) lost his keys.
My best friend, *who is a taxi driver, lost his keys.

3. A soap-opera is a program (It changes people’s ideas about real life).
A soap-opera is a program which changes people’s the ideas about real life.

4. A radio is a device (It lets people listen music and news).
A radio is a device that lets people listen music and news. (you added too many words) 

5. A restaurant is a place (There people go to eat).
A restaurant is a place where people go to eat. (Good job!) 

6. A computer is a device (It helps people to do different things).
A computer is a device *that helps people do different things.

(The "it" is unnecessary, because you are already saying "that," & "that" refers to the object--the computer. Also, I took out the "to" you could leave it in, but we don't say "to do.")

7. The chupacabras is a monster (It lives in people´s mind).
The chupacabras is a monster *who lives in people's mind. (Even though la chupacabra isn't real he/it is a "person")

8. A butcher is a person (He sells meat in a butcher’s shop ).
A butcher is a person who sells meat at a butcher shop. (Good!)

9. A church is a place (There people go for singing and praying).
A church is a place where people go for singing and praying.

(This is technically right, but the example is saying something wrong.. we don't say "for singing and praying" we say, "to sing and pray" if "where people go" was not in the sentence it would be correct, but with "where people go" it is implying that those people will receive the singing and praying... for it to be actually correct the sentence should read: "A church is a place where people go to sing and pray")

10. The hospital (my mother works there) is far away from home.
The hospital where my mother works is far away from home.