（レストランに一人で行ったとき入口で店員に）「一人です」って英語でなんて言うの？

When entering a restaurant without a booking you would say
"I don't have a reservation."
 If you are eating alone you would say
"Can I have a table for one?"
or
"May I have a table for one?"
 If you want to say this more informally then
you could say something like
"Just a table for me please."
or
"Table for one".

"One" Here you are answering when asked how many. You can accompany this one with a number one hand gesture.

"Just me" another way of letting the host ( this is what a person that sits you at a restaurant is called) know that its just you.

"Me, my self and I" this is a more relaxed, funny way of saying one person.

Host: How many?
You: Just me, my self and I! (smile)

The most casual and informal way to say this would be to simply state: "Just me"

The formal way to ask for a table for just yourself, would be to ask for: "A table for one"

When you are asked by the waiter at a restaurant, you can simply state that you are a party of one. Or, you can say that you are a single party. Most often, we will just ask for a table of one. When I dine by myself, I will advise the waiter or hostess that I am dining alone.

If, on entering a restaurant, you are asked how many people are eating, you may say:
"It's only me.'
"I'm dining alone.'
"Just me.'
"One."