There are two basic types to be aware of, 'Tuberous' and also ' Fibrous rooted begonias.
They are fleshy plants which can easily be damaged by frost, so you have to be happy that all danger of frost has gone, before planting out into the garden, otherwise they will look pretty sorry when the leaves turn white.
Once carefully hardened off, it should be safe enough by mid April in most parts of the UK.
A good way of getting the tuberous type started is by plantig the tubers in pots indoors or if you have a frost free green house or cold frame.
The tubers (corms) look like little bird nests and can be planted in pot of compost with the concave side up then water liberally.
The corms are usually available at this time of the year, in garden centres.