Well I reckon that you should wait for her to make the first move to whether you should progress further than first base because you don't want her to be uncomfortable and not be ready for what she's going to see or what's going to happen. She'll probably make the first move by slipping her hands under your shirt and nudge your shirt up higher than your waist.. then you can go ahead and remove it. Please DONT unbutton her clothes if she hasn't given you any signs that she wants her clothes removed.
There are no rules to this. usually what happens just comes naturally. there's nothing worse than if you say "ok, it's been 5 minutes of me being shirt-less.. now it's time for me to unclip your bra." Do you know what I mean? It'll just play out to how comfortable and how you feel with each other. There aren't any rules to follow.. you'll just feel it.