Have you done normal ancients? If not do them, and you should hit level 25. Then, as most level 25s do, you can start doing normal baal runs. From 25 until it gets slow norm baal runs is the fastest way to level. After norm baal gets slow, go to nightmare baal, and then eventually hell baal runs.
If you have done normal ancients, it is probably best that you get to level 25 before you start baal. I find having a friend do normal travincal and then mephisto council member runs are the fastest, but there are many other options.