Does hexproof prevent you from targeting your own creatures with your own spells?

Hexproof is defined in CR 702.11b: a permanent with hexproof cannot be the target of spells or abilities your opponents control. The key word is opponents. This means hexproof only protects against targeting by players who do not control the permanent — it does not stop you from targeting your own creatures with your own spells or abilities.

So if you control a creature with hexproof, you can still target it with your own instants, sorceries, or activated abilities. Your opponents, however, cannot target it at all, regardless of whether their effect would be helpful or harmful.

There is a rarer variant called shroud (CR 702.18b), which prevents targeting by any player, including the controller. If your creature had shroud instead of hexproof, you could not target it with your own spells either. Hexproof was introduced specifically to fix this limitation of shroud.

Concrete example: You control a Witchstalker (a creature with hexproof). Your opponent cannot target it with Doom Blade. However, you can freely cast Giant Growth targeting your own Witchstalker to give it +3/+3, because hexproof only restricts your opponents from targeting it.