You need to take your digital camera and lenses into Jessops or somewhere similar and see if they have an adaptor (I have one but it's for a wide angle digital lens) You will be able to check the focal length and see if the picture will look ok. If they have an adapter and it's not too dear you can buy it there and then. If it's expensive, make a note of the spec then look on the interweb.
The simple answer is YES (probably). What make is the SLR camera and what is the lens. You could end up spending a lot of money to buy a DSLR that is compatible with your lens. This is like buying a new car that will fit the wingmirror you received for xmas from your eccentric aunt.
any Nikon f-mount lens will mount on any Nikon DSLR. Canon I believe changed their lens design some years ago, so it depends. Sony Alpha's are a development of the Minolta DSLR/SLR range, so they are for the most part braodly compatible I believe. however BC has a point...you'd probably be far better just buying a DSLR kit with a basic lens, rather than basing your choice on the lens you have.