Wait until your daily login reward is a 75% discount, then buy some plat. (People can make plat off trading, but as a new player you won't have anything that people want.) With the discount, you can get something like 1000plat for about $10. That will be plenty if you only use it for weapon and frame slots. You will definitely want a variety of both of those; I've kept all of the frames I've built (even though there are only a couple that I use regularly), and I'm starting to wish that I hadn't sold so many of the weapons that I used to have.