to start yourself off I would recommend the following stuff

Enttec Open USB DMX dongle
5-3 pin converter
VSA
Freestyler (free download)
DMX dimmer

To control 20 floods you can chose to have 5 4 channel dimmers and control each light individually or wire your floods in chains of 5 and plug each chain into 1 of the 4 channel dimmer ports

You can then choose to control your lights by creating a sequence in VSA that controls the brightness levels or by using freesyler as a lighting control desk and manually controlling the channels.

To control your lights each channel will have an address allocated to it, with a dimmer pack this is done via a dip switch which allocates addresses to all 4 channels on the pack, ie if you set the dip switch to 100, then the 4 channels will be 100, 101, 102 and 103 (up to 512 channels are available)

You just then use the software to write a value from 0-255 to each channel with 0 being off and 255 being maximum brightness
, with VSA you can tie the process up to a sound file, so if you wanted thunder and lightening then you synch the flashing of the lights with a thunder track and hey presto you have TnL.