You can nearly guarantee you will sit together by selecting seats next to each other. If you are not eligible for free seat selection, you may have to pay. If you opt for tickets without selecting a seat (or paying) being on the same reservation doesn't guarantee a seat together, though you are more likely to be assigned at random to adjacent seats if they are available, and you may be able to change your computer-assigned seats to be closer.

If it is important you sit together, just pay for seats together.