@Mikaël Castellani People need to understand what your product does in 5 seconds or they'll leave. It needs to clearly identify who it's for and why they care / what benefit it provides to them. You also need very visible "call to action," i.e. what action you want them to do, i.e. signup button. And you want all of this "above the fold," i.e. visible to the user with no scrolling because it's the only part of your website you know everyone who visits will see.
For example, the end of your page should be the beginning of your page.