This treatise, such as it is, is based solely from my own experiences of learning to play on the lute after having been a guitar player for many years. I don't pretend to be an expert, but I hope that what I have learnt from making the transition will be helpful for those thinking of doing the same. Much of it is merely common sense, yet I often astound myself as to how often I manage to over look common sense when doing something for the first time myself.
When choosing a lute the most important thing to keep in mind is the style of music you are going to play with it. Put simply, where possible choose a lute modelled on the sort used to play the music you want to play. For music around the fourteenth century find an ude, for music from the fifteenth to late sisteenth century choose a six course lute. For music of the late sixteenth to mid-way through through the seventeenth century choose a seven or an eight course lute. After that you will have to look for an archlute.
If you are buying a lute second hand there are a few things you should be aware of. The tuning heads may provide problems check that they are firm and don't unwind under normal pressure. If they do unwind ask the owner if their is a trick to them (it may be a matter of gently forcing the heads into head of the lute). The frets should all be gut. Check that they are reasonably firmly in place (so that your left thumb won't displace them while playing) and that no buzzing occurs while playing any fret (which can happen if a gut fret is replaced by something unsuitable or a fret is far too loose).
Left hand technique needs to be better on a lute than on a guitar, partly because of the wider neck and partly because of the fact that most courses are double strung.
Don't let your thumb come over the top of the neck. This causes your fingers to become more paralell to the fret board and can cause them to interfere with the strings. Try to keep your thumb opposite to your middle finger. This is good guitar technique, but its need is more apparent on the lute.
Although not strictly necessary, some lutenists balance the little finger of their right hand below the strings. This helps to keep the lute balanced.