How can I align the first line of a description list with the rest in LyX?

Edit: The Description in the standard article class typesets as you desire, but the typesetting changes according to document class. So: Change your document class to the standard article, or redefine how the description environment is typeset.

Original: If you are working in the standard article document class, then the Enumerate environment typesets as you want.

So: Change the environment of your list from Description (?) to Enumerate.

If you want the items labeled with something custom, then go Insert > Custom Item when on the line you want to change the label of and then write your desired label in the resulting box. In the screenshot, I didn't bold the text, but bolding it works, too.