I'm creating styles for my book (to print) in InDesign. I'd like my "chapter title" to be somewhere lower on the page, not on the top, but can't do it. How do I set the paragraph style, in this case for the first paragraph – the chapter title – to add space before, since InDesign does not allow that?
For answers that are better for your question, a screenshot would be helpful. Something that helps others see what you're trying to achieve.
Create a separate Master Page for the chapter title page and let the main text frame start lower on the page. Adding space before a paragraph that is first in the text frame is not possible, as I recall.
In all fairness, what you are trying to achieve is more a "layout" task than a "typography" task, so I think it makes sense that adding space before a paragraph that is first in text box is not doable. It makes that paragraph style less useful.
Enable the Paragraph Rule: Above for your style. Set its weight to 0 and (you never know) its color to [None]. Check "Keep in Frame"; then, use "Offset" to move the paragraph down.
Apart from using the Rule Above (which you may need to, ehm, draw a rule above), the only drawback is that the Offset is calculated from the baseline of your paragraph. That means that to get an exact distance to the top margin – say, you want 10 mm of white above the capitals – you have to draw a horizontal guides first, and then align the text by eye.
One solution is to vertically offset the text negatively as much as you need and set a space after enough to compensate.
This isn't the cleanest of workarounds—To edit the text you need to select the text where it should be, not where you can see it, for example—but it works. A better option would probably be creating a new master page, as @herrklaseen's answer suggests. This option would work as-is though, without the need for changing master pages if your copy changes.