select how? onClick? onMouseOver?have you seen this sample? it demonstrates how to define a new symbol and pass an existing graphic to display in the map (on top of the existing graphics layer created from QueryTask results)
var highlightSymbol = new esri.symbol.SimpleFillSymbol(esri.symbol.SimpleFillSymbol.STYLE_SOLID, new esri.symbol.SimpleLineSymbol(esri.symbol.SimpleLineSymbol.STYLE_SOLID, new dojo.Color([255,0,0]), 3), new dojo.Color([125,125,125,0.35]));
...
dojo.connect(countiesGraphicsLayer, "onMouseOver", function(evt) {
map.graphics.clear(); //use the maps graphics layer as the highlight layer
//pass the event graphic geometry and second symbol
var highlightGraphic = new esri.Graphic(evt.graphic.geometry,highlightSymbol);
map.graphics.add(highlightGraphic);
});