CloudWatch collects metrics from AWS services automatically. EC2 reports CPU, network and disk I/O every 5 minutes; enabling detailed monitoring gives 1-minute data at extra cost.
EC2 does not report memory or filesystem usage, because those are visible only inside the operating system. A later post covers the agent that fills that gap.
Create a CPU alarm and send it to an SNS topic you have subscribed your email to:
aws sns create-topic --name ops-alerts
aws sns subscribe --topic-arn <arn> --protocol email --notification-endpoint you@example.com
aws cloudwatch put-metric-alarm --alarm-name web1-cpu-high \
--namespace AWS/EC2 --metric-name CPUUtilization \
--dimensions Name=InstanceId,Value=i-0abc123 \
--statistic Average --period 300 --evaluation-periods 2 \
--threshold 80 --comparison-operator GreaterThanThreshold \
--alarm-actions <topic-arn>Require two evaluation periods so a single spike does not page anyone. Also alarm on StatusCheckFailed, which catches hung instances.