Formal:
* आपको बताऊंगा/आपको बताऊंगी (aapko bataunga/aapko bataungi): This literally translates to "I will tell you."
* आपको सूचित करूंगा/आपको सूचित करूंगी (aapko soochit karunga/aapko soochit karungi): This means "I will inform you."
* आपको अवगत कराऊंगा/आपको अवगत कराऊंगी (aapko avgat karaaunga/aapko avgat karaaaungi): This means "I will make you aware."
Informal:
* आपको बता दूंगा/आपको बता दूंगी (aapko bata dunga/aapko bata dungi): This is a more casual way of saying "I will let you know."
* आपको बताता हूं/आपको बताती हूं (aapko batata hu/aapko batati hu): This means "I will let you know" and is often used when you are about to give someone information.
Other options:
* तुम्हें बताता हूं/तुम्हें बताती हूं (tumhen batata hu/tumhen batati hu): This is an even more informal way of saying "I will let you know" and is used with friends and family.
The best translation will depend on the specific context of your conversation.