c1. Old houses may be full of secrets because we may not know who (has?) lived there before.
Old houses may hold lots of secrets, especially if we don't know who has lived there in the past.
2.when you are wealthy, life is easier but I think everything could get bored (can get boring?)
When you are wealthy life is easier, but I think it could become boring after a while.
.3.the best will be if you call me (how to change this sentence?)
It would be better if you called me.
4.Famous people become automatically rich because the media create their wonderful image and they become celebrities who earn a lot of money (sounds clumsy)
Famous people become rich without effort because the media gives them publicity and their celebrity status generates wealth.
5.it seems a long time since we've gone out together (since we went out?)
It seems a long time since we last went out together. (although your examples are OK)
6.I haven't been to this new restaurant yet, but my friends said they served fantastic pizzas.
I haven't been to the new restaraunt yet, but my friends told me that they serve fantastic pizza!
After that we can do to the market square.
After that we can go to the market square.
7.It's the best to live in a small house
It's better to live in a small house.
8.living in a flat is really noisy.
Living in a flat can be really noisy