this is just a rough guide, but should work out fine:

• cd ˜
• openssl genrsa -des3 -out server.key 1024
• openssl req -new -key server.key -out server.csr
• cp server.key server.key.org
• openssl rsa -in server.key.org -out server.key
• openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt
• cp server.crt www.gosi.at.crt
• cp server.key www.gosi.at.key
• mkdir /etc/lighttpd/cert/
• cat www.gosi.at.key www.gosi.at.crt > /etc/lighttpd/cert/www.gosi.at.pem

now alter your lighttpd config accordingly and have fun:

$SERVER["socket"] == "IPADDRESS:443" {
ssl.engine                  = "enable"
ssl.pemfile                 = "/etc/lighttpd/cert/www.gosi.at.pem"
server.name                 = "www.gosi.at"          
server.document-root        = "/var/www/gosi.at/public_html/"
}

 
If you ever are in need of installing a paid certificate you might need a intermediate certificate, you can handle that this way:

$SERVER["socket"] == "IPADDRESS:443" {
ssl.engine                  = "enable"   
ssl.ca-file                 = "/etc/lighttpd/cert/thawte.pem"
ssl.pemfile                 = "/etc/lighttpd/cert/www.gosi.at.pem"
server.name                 = "www.gosi.at"
server.document-root        = "/var/www/gosi.at/public_html/"
}