A handful of very common verbs don't build the future on their infinitive. They have a special stem — but the endings stay exactly the same: -ai, -as, -a, -ons, -ez, -ont.

Infinitive	Future stem	First person
être	ser-	je serai — I'll be
avoir	aur-	j'aurai — I'll have
aller	ir-	j'irai — I'll go
faire	fer-	je ferai — I'll do, make
venir	viendr-	je viendrai — I'll come
pouvoir	pourr-	je pourrai — I'll be able to
vouloir	voudr-	je voudrai — I'll want
voir	verr-	je verrai — I'll see
devoir	devr-	je devrai — I'll have to
savoir	saur-	je saurai — I'll know

The impersonal expressions follow suit: il y a → il y aura (there will be), il faut → il faudra (it will be necessary, we'll have to), il fait beau → il fera beau, il pleut → il pleuvra.

Every stem still ends in -r. And in conversation, on often means we and takes the il form: on sera, on ira.