Hi Hilary

A good Community Tutor (or Pro Teacher) will lead you in terms of the conversation so you don't have to worry at all about thinking of topics to talk about.  As for nerves, a good teacher will be aware that you may be a little nervous and is sure to treat you in a very warm and respectful manner which will put you at ease.

One thing that you might wish to consider:  If your teacher does not ask you to specify, then you may wish to specify as to how you would like to be corrected during your lesson.  Some students like to be corrected immediately.  Some prefer to wait until suitable pauses in the conversation to review any errors.  You may also wish to specify the level of correction that you require.  Some students may only wish to be corrected on occasions where they have said something that would not be understood by a native speaker.  Others may wish to be corrected when they say something that would be understood but sounds a little clumsy/is not a very "native," way of saying something.  Some students want to have every mistake - no matter how small, corrected during the course of their lesson.

You might also wish to think - prior to your lesson - about whether (or not) you would like to do some homework as your teacher may ask you whether you would like some.  My suggestion would be that you consider asking for a small amount of homework, in between each lesson.