It depends whether it is in sun or shade and how much space there is. Any of the plants sold as alpines would be suitable, but avoid any invasive species (especially the one known as snow-in -summer, Cerastium tomentosum). Try to choose ones that bloom at different times of year. Also, put in some dwarf bulbs such as Iris danfordaie, cyclamen, and dwarf narcissus species. Small conifers are good for a while but make sure that they are very slow growing and not just sold as small plants.A good one is Chamaecyparis obtusa nana. Borrow a book about alpines from the local library and browse the pictures.

I would go along to the Garden Centre and look at their display of Alpine plants to see which ones you like. Wait until they are in flower as the pictures on the labels are often inaccurate.
Campanulas, thrift, saxifrages and gentians are good ones to start with. Bulbs certainly but I would question Iris danfordiae as it usually just flowers the once. There are many other dwarf irises to choose from - and don't forget a few of the early crocus.

Snowdrops will give you your earliest flowers possibly followed by winter aconites. You can buy rhododendrons that take a long time to get to any size. In my experience if something is described as 'choice' it probably means it isn't as easy to grow :-)

As Volunteer suggests a book is useful... personally I think it is better to but a book rather than borrow from the library, a good starting point would be 'A Rock Garden Plant Primer' by Timber Press.

thanks

I tend to borrow the books then buy the best ones! Also, there are often older books in the library, shops only seem to sell the latest trendy ones.

Try the second hand bookshops, Volunteer. We have a large one in Inverness that has a huge gardening section and often has some of the classics. As you say, the books in the high street shops are generally poor and glossy. The one referd to by Moongrower would probably have to be ordered on-line as it is too specialist for the high street. However, as book seller to the SRGC members, I can recommend it as the best, new book on general rockery plants.