Going to a nice restaurant.

There will be a host or hostess behind a counter, who will ask the newly-arrived customers how many are in their group, then he/she will lead, or guide (not sure what's the best word) show the guests to the table (in some places there are booths; the host(ess) you will be asked if you prefer one), then inform you that a waiter(ress) will soon come to take your orders. The best option to call for addressing a hostess is to use the word "miss", no matter her apparent age. (there's some debate about that: miss can be belittling)

The trickiest part about writing and correcting this is keeping gender-neutral without looking messy.  In modern English, some words have been accepted as gender neutral, eg. waiter may now refer to either a male or female.

Client has a slightly different meaning in English - it usually means there's an ongoing business relationship, ie. about negotation and money. For people in a restaurant, we call them guests or customers (when counting bookings, they're numbered as heads or pax). However, we do refer to the whole customer base of a restaurant as the clientele.

Sorry if that all looks a bit confusing - this was quite a good exercise on your part. :)

I am going to a restaurant.  There will be a host or hostess behind a counter, who will ask the newly-arrived clients how many are in their party.  Then, he/she will guide them to the table or booth equipped to accomodate the number of people in the group.  A waitress will come to the table to take the orders.

It is always best to call a hostess "Maam", no matter how old she is.

Good start, Flavio. 

  Going to a nice restaurant.

There will be a host or hostess behind a counter, who will ask the just arrived clients how many they are; then he/she will lead, or guide (not sure what's the best word) the clients to the table (in some places there are booths - the host(ess) will ask if you prefer one), and say that soon a waiter(ress) will come to take the orders. The best option to call a hostess is using the word "miss", no matter her apparent age.

Going to a nice restaurant.

There will be a host or hostess behind a counter, who will ask the just arrived ('newly arrived' is better) clients how many they are; then he/she will lead, or guide (not sure what's the best word - both are fine) the clients (them) to the table (in some places there are booths - the host(ess) will ask if you prefer one), and say that soon a waiter(ress) will come to take the orders. (order) The best option to call a hostess is using the word "miss", no matter her apparent age.

In a nice restaurant, newly arrived diners are likely to be met at the entrance by a maitre de who will ask for the size of their party and show them to a suitable table. A member of the waiting staff will take the order.... etc.