Young Avengers can be split into two series, but of which make great starting points for new readers.
If you need help deciding which is for you, Kieron Gillen, the writer of the second series, described the difference between the original and his series like this: "Young Avengers in its first incarnation was about being sixteen. This Young Avengers is about being eighteen. The difference is telling and profound. At sixteen, it's still about wanting to be in the adult world. At eighteen, it's about being in the adult world whether you want to or not."
The first book of Young Avengers, and The Children's Crusade:
All three books of the second series: