I'm a newbie when it comes to mapping application. I had successfully loaded my OpenstreetMap data in a Postgresql/PostGIS database. Now my problem is how to render the vector data (the POI's, ways, etc) from the database to the the map. I don't have the slightest idea on how to do it, though I think I knew how they are rendered (OpenLayers use KML/XML files to render vector data, I tried it in their tutorial). Can someone explain to me how it works; do I need to get the data from the database as XML file? I don't know if I'm missing here something. Please help me. Thanks a lot!
If you want your web map to look like the one shown on openstreetmap.org, you can use the same renderer they use: Mapnik. It can read from PostGIS or the original .osm files (would have saved the effort of importing the data to PostGIS first) and some other formats. I'm not sure how easy it is to use Mapnik though. Some information on rendering and customization.
It really depends on what the result is supposed to look like and how much effort you're willing to put in it.
For Mapnik rendering, see http://www.weait.com/content/build-your-own-openstreetmap-server, after look for creating tiles on http://wiki.openstreetmap.org/wiki/Creating_your_own_tiles and http://wiki.openstreetmap.org/wiki/OpenLayers_Simple_Example for displaying result in Openlayers.
For Mapserver, http://trac.osgeo.org/mapserver/wiki/RenderingOsmData
In all cases, some days of working to get results.
GeoServer is a Java webapp that will require a JVM and Java application server, such as Apache Tomcat, to run. As a Java developer, I prefer GeoServer to the other WMS's because of my development and deployment process, among other things such as Open Geospatial Consortium compliance for it being a WMS and WFS.
GeoServer does not read OSM data files natively, but PostGIS is the preferred way of dealing with large amounts of data in GeoServer. There are some predefined styles available in a default GeoServer installation, although I personally like the one provided in Ivan Williq's blog post.