Start with Blink. It's a very famous and popular episode with Tenant and it's basically a standalone. It's an especially good entry point as the Doctor is barely in it.

You might get lost a bit with some story threads. If you really have no intention on watching the previous doctor I would watch a season recap video online. That way you're familiar with the important bits of lore and story lines and whose who but it shouldn't be too bad. They don't really delve too much into all the old doctors lores and stories, and when they do they refresh your memory, so at most it's one season of stuff before tenants.

That's where I started and still really enjoyed it a lot. Definitely a character here or there that they assumed I knew but for the most part it all made sense.