That severly limits your options. (Which reminds me, you might want to state a major detail like that up front next time.)
The straightforward solution would be to apply your bold font (or, better: a character style) to the first line of each paragraph using the .lines property of a paragraph. Uh -- customary link to the HTML version of the OMV omitted, it seems I don't go back as far as CS2 ... (There is a CHM for it at http://www.jongware.com/idjshelp.html).
This ought to work:
app.selection.paragraphs.lines.appliedCharacterStyle = "YourBoldCharStyle";
... where your cursor is inside the paragraph you want to change. Adjust at will to cater for your special needs.
There is a downside to the straightforward method, though. Unless you are using a special font which has bolds with the same width as its regular (quite unusual), the text will reflow after you applied it. So then there are one or more bold words on the next line, and you should remove the bold char style from those. But! At that point InDesign may decide those non-bold words can fit again on the first line!
Well-written code does not help, as it might be possible the same word oscillates back and forth with every change between the first and the second line, each time getting it wrong ... So at *some* point you are going to have to help poor ID a bit; perhaps by fixating that first line with No Break applied to it.
thank you very much for your help. I did figure out how to do it finally after a lot of "fiddle"-programming . The link you also pasted was very usefull cause the reference CHM was a boon.
well my basic algorithm was (vbscript)
place some text
app.paragraphs(1).lines(1).appliedFont = "somefontname Bold"
And the second thing about the words being wrapped to second line, well i guess i guess the easiest would be to make the second line non bold after the first one is made bold, hacky i know but "just works"!
thank you very much Jongware, harbs