You seem confused about how GeoTools works, you can make a
getMap request using the
gt-wms module to fetch an image of a map from a WMS server (like GeoServer, MapServer or QgisServer). If you are more interested in actual vectors then you could use the
gt-wfs-ng module to make a
getFeatures request to a WFS server (all the previous mentioned servers support WFS too).
However if you want to fetch vector data from a datastore (e.g. PostGIS, Oracle, Shapefiles, GeoJSON files etc) you will need to create a datastore of the relevant type. This will allow your program to read the data and allow you to carry out analysis or renderer it to a map. The GeoTools quickstart tutorial covers this.
However, from your other questions, I suspect you are trying to write a new WMS server from scratch in which case you will need to fetch the data from PostGIS (using a PostGIS datastore), reproject (CRS modules) it if necessary and then render it to an image (see the rendering module) having taken any styling into account (see the styling module). You will also need to write some code to parse and understand the WMS getMap requests that you receive (for some pointers you can look at GeoServer's GetMapKvpRequestReader).