Learn French: Je suis décidé à apprendre le surf aujourd'hui.

This is a major trap! The verb décider uses de (as in j'ai décidé de le faire). However, when you use it as an adjective with être (meaning 'to be determined'), the phrase être décidé requires the preposition à before the infinitive.

Is there a difference between Je suis décidé à and J'ai décidé de?

Yes, there is a subtle difference. J'ai décidé de means 'I have decided to', which focuses on the past action of making a choice. Je suis décidé à means 'I am determined to', which focuses on your current state of mind.

If a woman is saying this sentence, does décidé change?

Yes, it needs an extra e. Since décidé functions as an adjective here following the verb être (to be), it must agree with the subject. A female speaker would write Je suis décidée à apprendre. The pronunciation remains exactly the same.

Why is the sport of surfing translated simply as le surf?

French frequently borrows English words for sports and turns them into standard masculine nouns, dropping the '-ing' ending. When talking about the sport, French just uses le surf. You will see similar patterns with le skate for skateboarding.