Same advice re knives as below - but even a good folder works well
As to woods I like Lime and dislike Pine, Ash is pretty good and Oak while green carves well once it is seasoned it is realy hard work - the same goes for Holly.
Strangely enough Ivy - if you find a good thick bit is wonderful to carve when green but dries like iron and "wrinkles" as it dries with interesting effects