I wrote serons because "on" referred to

Hi Louise,

I am guessing at what your question might be because your post didn't show one....

On sera à la fête ce soir. -- We will be at the party tonight.
Nous serons à la fête ce soir. -- We will be at the party tonight.

Both sentences mean the same thing. The first version, with "on" is a bit more colloquial than the one using "nous", though. Note that the verb has to fit the subject in person and number. Since "on" is third person singular, that's what you need for the verb: sera. When using "nous" as the verb, you need first person plural, hence "serons".

-- Chris (not a native speaker).

When using serons you use the subject pronoun Nous for plural 

But if you use the subject pronoun on you use sera

It’s a quirk of the language, ‘on’ is usually treated the same as ‘il/elle’ so sera would be correct.  See Il est, on est, the same.