We've collected an incredible amount of advice for new angel investors over at AdviceForAngels.com
On that site, you'll find lots of resources that you can learn from, including:
1. How to be an Angel Investor, pt. 3
This presentation contains my best advice for new angel investors.
2. Interviews with other angels:
Video interviews with successful investors sharing their best advice.
3. Recommended reading:
Links to other resources that offer expanded or different opinions than I do.