Welcome to WAMAS.
A few things:
1) Nitrate, Nitrite, PH,and Ammonia test kit (just get the cheap API one and use it for reference) - around $30 or so (almost all stores sell them, even some of the bigger chains).
2) Decide on what you'd like to do with the tank (fish only, corals), what type (of fish and/or corals), their temperament and the like (research on what others have experienced with their livestock).
3) If you'd like to save money in the long run look for a RO/DI unit - around $100-$150
4) Be patient take your time, don't rush just because something looks cool or is on sale (this is typically for livestock)
5) Although not a necessary to have, maybe look into a controller (Neptune Systems or Digital Aquatics) to automate certain things (lights, heater control, top off) it could help you out some.
6) Maybe a refractometer - around $50
Since this is a new tank:
1) Think about what you'd like to have as your clean up crew (snails, hermits, etc). These are usually the first inhabitants after the cycle.
Also, join WAMAS, they're a pretty cool bunch (and for $20 it's a steal). As a newbie to the hobby, some of the folks may even help you out with some easy beginner corals. You also get to unlock the Sale section (which could save you a lot of money).
Another thing you may want is to put in your location, not a necessary, but good to have sometimes.