Take your time. If she’s into you, there is no reason to rush immediately into getting into her pants. She’ll still be there next week.
Take a few weeks to get to know the office, learn some of the power dynamics, and make sure you are not unknowingly walking into something more complicated than it looks. You don’t want to be making out with her at the Christmas party, only to find out that your boss is her ex and she’s trying to make him jealous by jumping the new guy.
Once you have some background, ask her out to dinner or drinks after work. Keep it casual, along the lines of “Hey, next Thursday I was hoping to check out that new Thai place opening down the street. If you are free, I’d love some company. I hear their Pad Thai is amazing.” If she accepts, she is almost certainly interested. If she declines but offers to make concrete plans (“I’m not a big Thai fan, but I’m free on Friday if you want to meet up,)” then she is probably interested.
If she declines and gives vague plans (“Sorry, I have tickets to the Ice Capades that day. But we should, uh, totally meet up one day,”) then she is either not interested in you, or only interested in you as a backup/flirt friend/distraction. You can have one more shot a couple weeks later at making plans with her before it gets creepy, but don’t get your hopes up. After that, she knows you are interested and if the feeling’s mutual she’ll make the next move.
Remember that it’s not unusual for people to flirt at work, even if they are not actually interested in escalating things, because it provides a nice distraction from the tedium of office life. Everyone likes attention, and some people seek that attention in the workplace because you can spend a lot of time with someone flirting, but it’s easy to keep it from going any further. It’s the same reason as why some women cultivate flirty relationships with gay guys. It’s fun and it’s “safe.”
Dating in the workplace isn’t inherently bad, it’s just that it has so much potential to go massively wrong. Most relationships break up, sometimes rather quickly, and you can never know exactly how someone is going to react. Even if you are mature, she may end up being nuts and that could be a lot of trouble. And, frankly, we are poor judges of our own maturity. Even a piece of granite would have trouble keeping it together in close quarters with an ex that they loved who is dating other guys and moving on.