If you haven't already entered the child in question, add them as a "New Unlinked Individual" - Ctrl+N on the keyboard, or Add > New Unlinked Individual via the menus.
Next, add the parent you know about to this person. You'll already have him or her in your database, I expect, so instead of adding a new person, choose to link to that existing person.
At this point, your child has only one parent. If that parent already has the subsequent marriage in place, that spouse is already set up to be the step parent to the child.
You can display the step relationship on your screen - right-click on the child (any child, in the child position in Family View), hover the mouse/cursor on View in the menu that appears, and click on "Show 1/2 kids".
To make this step relationship appear in reports, you'll need to add the step-parent as another parent to the child. Children can have any number of parents in Legacy!
Once you've added the parent, right-click on the child (the specific one this time) and then click on Children's Settings. You can then specify the nature of the relationship to either parent, and therefore show that one is a step parent (or whatever you want it to show).
Does that do what you want it to do? I think I've covered most of the basics here, so whatever you want to do (or not have showing) should be adjustable with these settings. Unlink any parents from the child if you don't want them showing there. Or tell us a bit more detail if you still need help with it. :-)