Assuming you have your document defined as oDoc, the following code should get you the desired result:
Word.Paragraph oPara = oDoc.Content.Paragraphs.Add(ref oMissing);
oPara.Range.Text = "hello planet earth here's what I want to do";
object oStart = oPara.Range.Start + 13;
object oEnd = oPara.Range.Start + 18;
Word.Range rBold = oDoc.Range(ref oStart, ref oEnd);
rBold.Bold = 1;
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