Yes, you have to take the old stitch off the L needle, keep the new st on the R needle. You won't have sts in the 'middle' they're either on the L needle you haven't knit yet on this row, or the new ones on the R needle. You can look at the Knit st videos here, and the ones at the site Jan linked to. Also look on the Tips page for Demo of a Small project which shows casting on, working a few rows in stockinette st, and binding off so you see what all the steps look like put together. Just keep working across the row making stitches. After a few rows your tension should tighten up. You might be using a thin yarn with very large needles which would also make loose stitches.
sue- knitting heretic