what should I know about the clitoris before I have sex?

In my experience, the clit is the key to her enjoyment. Every girl is different, so you have to pay attention to how they react to how you touch it. Some girls like light slow touch, while others like very fast flicks. Some like light slow touch at first, then very fast intense flicks towards the end. I've had girls that need a lot of stimulation, and girls that find a lot of stimulation overwhelming. Listen to their moans and how their body moves. If you get a girl that doesn't show a difference either way, then ask her. If she is still being secretive, then don't worry about it too much and just enjoy your time. Don't go directly for the clit at first. Play around with the pussy area first, slowly getting to the clit.

am I right in assuming my partner will tell me what she wants me to do?

It's been my experience that most girls will guide you indirectly. Very few will tell you exactly what they like. If you get one that does, you have won the lottery. Otherwise, they will guide you indirectly.

In general, just have fun. Sex is naturally enjoyable. Above all, communication is key, but some people feel uncomfortable with being direct about it, so they communicate indirectly and that's okay. Listen to the language they choose to use. Regardless, don't take sex too serious, and you'll be okay. Good luck!