likewise i am in my very first grow from seed.......here goes

aluminium foil is pretty good to use......you don't need to paint the walls white.....just use it from the dull/dump side....otherwise you will create hot spots that are going to burn the leaves of the plants.....aluminium foil reflects around 75% of the light that it receives...
secondly, i would suggest to buy a 150 watt HPS and with the use of the chains you can adjust the height while the plants grow..floros are ok but they don't emit such big amount of lumens.....
finally, use a fan blowing directly cold air on to the plants because a)it will get hot will the HPS lamp and b)it will help them grow faster&stronger ..... don't put it on max speed though....you don't need a wind like a typhoon, just a wind that shakes their leaves and at the same time circulates air