The best soil for plums is a neutral to slightly acid, with a ph of between 5.5 and 6.5. As for the fertilizer question, enrich the soil before planting with composted material, compost from your own heap will be fine if you have one, then plant, and fertilise when growth begins, usually April/May.

Bamboo's spot on, but don't be too worried if you think your soil might be on the alkaline side. The native prunus cerasifera (cherry plum, myrobalan) is seemingly happy on chalk soils, as is blackthorn. Most important is good cultivation and a good start.