Step-by-step explanation:
Something that you know:
1. a lot about something.
2. that you have interest in.
3. what you're creating (because if you don't know, you wouldn't create a website.)
4. you're going to be active in.
5. you like, or you can quickly answer a question right away.
For three topics, I could suggest:
1. Learning websites
2. Helping the world (blogging - bored, channels/website - helps everyone not be bored, how-to - helps the world learn, simply like learning websites, etc)
3. Daily/private things/blogging.
Summary:
What you enjoy. Other than that, above is some ideas you may enjoy. Creating a website takes days or weeks, or sometimes years. However, if you are looking forward to it, enjoy yourself, and do it!