Convert a shapefile (.shp) to xml/json
What dassouki said. Get GDAL from http://www.kyngchaos.com/software:frameworks. Use it to convert a shapefile to GeoJSON like this:
$ ogr2ogr -f "GeoJSON" output.json input.shp
eg
$ ogr2ogr -f "GeoJSON" /tmp/world.json world_borders.shp world_borders
$ cat /tmp/world.json
{
"type": "FeatureCollection",
"features": [
{ "type": "Feature", "properties": { "CAT": 1.000000, "FIPS_CNTRY": "AA",
"CNTRY_NAME": "Aruba", "AREA": 193.000000, "POP_CNTRY": 71218.000000 },
"geometry": { "type": "Polygon", "coordinates": [ [ [ -69.882233, ...
...
GDAL is your friend. Use the ogr2ogr libraries, and here's a cheat sheet.
If you want a good GIS application, install QGIS and download the gdal converter plugin