#Points.1 {
  stroke: #d73027;
  stroke-width: 6;
  stroke-linecap: round;
}

<sld:FeatureTypeStyle>

    <sld:Rule>
      <sld:Title>Start</sld:Title>

      <ogc:Filter>
        <ogc:PropertyIsEqualTo>
          <ogc:PropertyName>id</ogc:PropertyName>
            <ogc:Function name="env">
              <ogc:Literal>lowestID</ogc:Literal>
            </ogc:Function>
        </ogc:PropertyIsEqualTo>
      </ogc:Filter>

      <sld:PointSymbolizer>
        <Graphic>
            <Mark>
                <WellKnownName>square</WellKnownName>
                <Fill>
                    <CssParameter name="fill">#FF0000</CssParameter>
                </Fill>
            </Mark>
            <Size>6</Size>
        </Graphic>
      </sld:PointSymbolizer>

    </sld:Rule>
 </sld:FeatureTypeStyle>

SELECT
  id
FROM
  tableName
WHERE
 id = (SELECT min(id) FROM tableName)
;

http://yourGeoServer.com:8080/geoserver/wms?
request=GetMap
&service=WMS
&version=1.1.1
&layers=yourLayer
&styles=yourStyle
&srs=yourSRS
&bbox=yourBoundingBox(normally determined by Leaflet/OpenLayers)
&width=256
&height=256
&format=image%2Fpng
&env=lowestID:yourVariableHere;highestID=yourOtherVariableHere