There are several approaches to this problem depending on (a) whether you
will have a TOC and (b) whether you want both the chapter number (and label)
and the chapter title in the TOC:
1. As Luc says, you can press Shift+Enter after the chapter number, then
type your chapter title. Assuming that you are using auto numbering with
"Chapter" as part of the number format, select "Nothing" for "Follow number
with." The line break in your chapter number-title paragraph will be
interpreted as a space in the TOC, so you'll have:
Chapter 1 Introduction
2. You can also use two separate styles, one for the number and one for the
title. If you do this, it's usually best to use Heading 1 for the chapter
title and a custom style (numbered) for the chapter number. In the TOC, you
apply numbering to the TOC 1 style to give this result:
1.<tab>Introduction
or
Chapter 1<tab>Introduction
or whatever you want to do with the numbering.
3. Alternatively, you can use separate styles for the chapter number and
chapter title and have both included in the TOC, in which case you'll get
separate lines for the number and title, and you'll probably want to omit
numbering for one of them.