Yes, everything that Last says, be honest about your abilities and don't worry about saying that you are nervous either, much better to be upfront than blase or overconfident. At every step from arriving on the yard to actually going out be fully prepared to back out if it doesn't feel right and safe, even if this puts the lady to mild inconvenience, look out for number 1 and don't be persuaded into anything beyond your competence level to suit what other want.
In terms of tips for when actually riding, I know it's easier said than done but you need to force yourself to be relaxed and almost floppy through your entire body, if you are carrying a lot of tension in your arms or seat or legs the horse will pick up on it and it may make him tense or joggy and this in turn will make you more tense too. Ask the owner of she can put a neck strap on (easy to make one from a spare stirrup leather) and if you feel the need to hang on, hold onto that rather than the reins, most privately owned horses are a lot less tolerant than riding school ones of being pulled in the mouth so if you feel unbalanced try not to pull on the reins. If there are 3 or more of you riding ask to go in the middle and a quiet horse should then just follow nicely on a loose contact without you having to do a lot of steering or leg, if you do need to put an aid on start very gently until you've gauged his reactiveness/sensitivity, the RS horses will likely have taught you to do a big kick to get going and a big pull to stop or turn but never do this on an unknown horse (even a RS one) as you could get much more of a reaction than you were counting on! I find talking to someone else, or to my horse, or even singing does the trick to relax me and make sure I remember to breathe...
It's very important to make sure the others in the group are sensible and follow good etiquette i.e. only trot or canter when the whole group agrees, stay in order, no overtaking unless forewarned and agreed, steady canters only or none at all if you have a nervous rider or green horse with you. I know it's a bit awkward but I'd check this is the shared understanding with the whole group not just the owner you are going to see, before setting out and make it one of my boundaries not to go at all if they don't explicitly agree, there's a few people on my yard frankly I would avoid hacking out with at all and particularly not if I had someone remotely nervous or novicey or just of unknown ability with me, even on my own very reliable horse, because they think no hack is complete without several fast canters regardless of ground conditions or other circumstances and are given to taking off very quickly side by side with little or warning which is just frankly unsafe, I'm a pretty secure rider but what if I was fiddling with my stirrup and not ready to go, or a walker with a buggy or dog appeared round a corner, an accident could easily occur. Sadly my experience is these types are to be found on most yards so be assertive and prepared to turn back if your boundaries aren't stuck to. That being said they are the minority and most people are absolutely lovely and very considerate and I'd have thought once you get going you'll love it, there's nothing like a nice hack on a sunny day to put you into spirits, enjoy!