To answer your question...
When your Nitrite and ammonia levels are at zero, (which can take 2 months, so be patient!), a 50% water change is a good idea, as you will probably be reading around 20 to 40 with your nitrates. This will assist to bring it down to about 30. You want to keep doing reasonably big water changes when it creeps back up to 40-45. IMO Dont let it get higher than this. Some soft corals probably will be unhappy!
Nitrates are ok in you tank for the first six months so long as you are not putting anything in your tank that requires superb water quality.
Most fish will handle and soft corals may actually enjoy the nitrate levels.
The only way to then get your levels down without spending heaps of money on apparatus is to put as much live rock in as many places as you can. Anearobic bacteria grows in the rock and reduces nitrate levels enough to start putting in the more delicate things. Fill you tank, your sump and everything else you can. Better to do it now whilst the tank is still cycling really, but not imperative.
This anearobic bacteria takes the longest to establish and settle and this is why six months is about right. Every tank will be slightly different of course.
HEAPS OF LIVE ROCK!!
Hope this helps.
Good luck!