Here are the basic tips for traveling alone in Europe:
Give Someone Back Home an Itinerary: List dates, flights, where you will be staying, and contact information for hotels.
Have an International Phone Plan: It's only about $10 a day. A wise investment for staying in touch.
Be Careful on Public Transportation: Take the bus, not the metro. Only use licensed taxis from a taxi stand or called by the hotel. NEVER take a ride from someone who solicits you.
Carry a Copy of Your Passport at All Times: Write contact information on it, medical conditions and prescriptions, hotel information.
Insurance: Always have travel insurance.
Go on Group Walks: Many cities like London offer informative and worthwhile group walks. See www.walks.com.
If Meeting Strangers, Do So in Groups: Years ago when I traveled alone, I would meet up with other groups of travelers for dinners, but never with a group of less than 5.
Always Be Aware: As I said, I always felt safe in Europe, but I was smart. If I found something suspicious going on, I got out of the situation right away by going in a hotel or restaurant. I avoid being out at night unless I was with a crowd.