Target

Whenever you attempt to interact with another creature or object, you declare your Action linked to that target or group of targets.
Ranged Attacks and many spells can target distant creatures or objects (see below). However, unless otherwise noted, in order to target a creature or object with an Action:

• You must be able to sense the target, either by Sight, Hearing, or touch.
• The target must be within your Reach.
• To interact with an unwilling target, you must typically roll a successful Attack roll.

Spellcasting Targets

A typical spell requires you to pick one or more targets to be affected by the spell's magic. The spell's description tells you whether the spell targets creatures, Objects, or a point of origin for an area of effect.
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless a spell says otherwise.

A Clear Path to the Target

To target something, you must have a clear path to it, so it can't be behind total Cover. If you place an Area of Effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

Targeting Yourself

If a spell targets a creature of your choice, you can choose yourself, unless the creature must be hostile or specifically a creature other than you. If you are in the Area of Effect of a spell you cast, you can target yourself. Areas of Effect Spells such as Burning Hands and Cone of Cold cover an area, allowing them to affect multiple creatures at once.