Short answer
If you already feel thirsty, you're already mildly dehydrated — that's normal and treatable at home. Go to the ER if confusion, dizziness when standing, or a rapid heartbeat show up.
Mild: headache, dark urine, fatigue — drink water, rest in shade, stay home. Moderate: no urination for hours, dizziness when standing up, confusion — get it checked. Severe: rapid heartbeat, confusion, fainting — ER now. Quick self-test: press a fingernail until it turns white, then release. If the color takes more than 2 seconds to return, you're dehydrated.