A logo is a unique and singular representation of your brand, it should be able to sum up your entire brand identity in one single simple image. So before making a logo for you, the designer should be able to get to know your brand identity.
So when designers ask you to tell them what you want, they are not asking you to describe the logo in detail. They are asking you to limit their options, by answering some fundamental (branding) questions, like:
- What is the brand/website about?
- What is the Tone of Voice of the brand/website? Do you want it to be playful or serious?
- What are the brand colours?
- Are there things you want or do not want associated with the brand? Do you want a logo similar to your competitors/peers, or something that is noticeably different?
- Do you want the brand name in the logo or not? Should it be a unique lettertype? Or should the logo be recognisable without any text?
If you cannot answer these questions yourself, it is too early to go to a designer for a logo. First think about your brand and what you want to be about, who your target audience is, how you want to speak to them, how you want to be perceived, etc.
These are branding decisions, and while some designers might also be able to help you with that, this is more marketing-oriented and possibly a lengthy and expensive process.
In this branding tutorial for example, designing a logo is only step 6 of 7. And here and here are more great guides to building a brand.