You can never go back.
AC1 is not a bad game on its own, but if you think you ever want to finish it in the future I implore you not to start the second one. The second one, like others have mentioned, smoothed out the rough parts and made AC2 a much more enjoyable experience. Unfortunately that makes it sound like AC1 is crud. Of course it isn't, if it was AC2 never would have made it to market.
If you play AC2, if you even watch somebody else play AC2 your enjoyment of AC1 will be lessened and you will find playing it to be a slog through the slow parts.
If you are the type of person who likes playing a game through, don't do AC2 first. If you're not, skip AC1.
There's also a DS game, which I understand is a prequel to AC1 but I've never played it.