That's great news that you've wanted this for ages and now have an interview. Don't let that overwhelm you with nerves though; just try to remember that they obviously think you're a good candidate, so its cliched but just be your normal self, iykwim?
Is it for older adults with functional mental health conditions (depression/anxiety etc) or organic mental health probs (dementia, Alzheimer's etc)?
Community posts tend to require you to demonstrate a good awareness of risk assessing and safe working practices (notifying colleagues of whereabouts, using electronic diary, ensuring there's a risk assessment in place before going to a clients home, utilising a personal alarm etc).
They'll prob want to hear about scenarios where you've: a) had to think quickly on your feet to problem-solve, b) made a positive difference to somebody, c) dealt successfully with conflict/a challenging situation.
If you can think of a few good examples to use as scenarios, that'll really help. They don't have to be from your working life, they can be from general life/family situations etc. As long as they evidence your transferable skills in those areas.
Older adults support/health work is so dependent on warmth, a down-to-earth approach, empathy, understanding, patience and genuine respect, so if you can let these aspects of your character come across at interview it will stand you in good stead.
You might possibly (dependent upon the job Band) be asked something about current challenges to the health/care sector (ageing population, pressures on inpatient beds, increased emphasis on care-at-home, social care funding cuts etc).
Above all, try and enjoy the interview as your chance to shine and let your keeness and passion come through....and the very best of luck! 
Will you let me know how you get on? 