Assuming the new series, I'd start with series 1, Episode 1 "Rose" and go from there.
The new series does a good job as a primer for Doctor Who. It's not comprehensive, and there are bad episodes, but still. Eccleston's run as the Doctor was INTENDED to introduce a new generation to the Doctor, who had been off the air for most of or all of their lives.
Classic Who is kind of a clusterfuck. Not only do the serials kinda mandate something like Wikipedia to do in order, some of them were destroyed, and aren't necessarily there. Or complete. There are entire arcs which are pretty awful, though I don't think it should reflect badly on the show. It spanned two decades of continuous airtime before being put on haitus, so there were bound to be missteps. Some of them big.
If you like the new stuff, going back is good. Everyone has their favourite Doctors (The Fourth is routinely heralded as the best, but I don't like Baker and prefer the second [what little I've seen] and the fifth to him, so it all breaks down to preference), so it's hard to say which one you would like best. Check pilf's list, as it's pretty much as good as any I can think of, and see which actors you like.
RoBi3.0 said:
In random some what related news, Christopher Eccleston the 9th Doctor was offered the role of Sylar in Heroes, but turned it down because he is against the stereotype of Brits, being super evil.
Probably for the best, I liked the guy they got to play Sylar.
I don't picture Eccleston in that role very well, though I will sing the praises of Eccleton otherwise.