The first season of the new series is the one with the 9th Doctor (Christopher Eccleston), who has Rose Tyler for his companion and changes into the tenth Doctor in the last episode of the season. You've seen that one, The Parting of the Ways. The next three seasons (2,3 and 4) are all with the 10th Doctor (David Tennant), but with various companions: Rose Tyler (season 2), Martha Jones (season 3) and Donna Noble (season 4). The 10th Doctor episodes you've seen are all from Season 3. I hope this helps a little.