Hi Randi.
As Eduardo said, you could say “le haré saber”
Also “se lo Haré saber” (formal), “te lo haré saber” (informal).
You can also use verbs “informar” and “avisar”
“Le informaré/avisaré”. “Te informaré/avisaré”
Informaly, you can use Simple present as future tense and people understand (depending on the context)
“Yo te/le aviso”
“Yo te/le informo“
Hope it helps
Let me know if you need more help
(Ie: “ok, yo te aviso”)