Yes, you can start a sentence with "However." It is often used as a transitional word to introduce a contrast or exception to the previous statement. When starting a sentence with "However," it is typically followed by a comma.

Example:

• I wanted to go to the park. However, it started raining.
• The project was challenging. However, we managed to complete it on time.

Using "However" at the beginning of a sentence is perfectly acceptable in both formal and informal writing, as long as it is used to show contrast or contradiction.