Hi. Welcome to the site.
I've not worked specifically with learning disabilities myself but in the support worker interviews I have had they have not been too demanding in nature, they are mainly about common sense. There are lots of posts on this site.. if you do a search with regards to support worker roles. Saying that, although a lot is common sense I still do a bit of prep each time because common sense isn't exactly always at the tip of my tongue when I'm nervous!
Questions will probably be fairly standard (why you want the job etc), skills that might make you suitable to the position. They might ask you scenario type questions. Proffessional boundaries are often a good thing to think about. They might ask you about previous experience. It is important to read the job description so that you have a clear idea about what this is, make sure you can vocalise this if asked about what your understanding is of the role. You might be asked about your feelings towards certain things you might be asked to do..perhaps personal care? Also perhaps thinking of any questions before you go for the interview..something that demonstrates you've engaged with the role or the place of work.
With regards to other preparation, one of the best things I ever did was request to go and view the unit before interview. Often you get to meet the client group and see if its something you really want to do..you kind of might get a bit of a free speech from someone that works there (might be handy in the interview for kind of inside knowledge).
I hope this helps. Good luck