It really depends on your current level of knowledge on fishkeeping. You need to know a few basics about water chemistry...the nitrogen cycle, pH, oxygen levels and salinity.
You will also need to know about fish compatability....nearly all marine fish are highly terratorial and intolerant of their own kind....some are even intolerant of fish the same size, shape and/or colour as themselves.
There are lots of different ways of keeping marine fish....and everyone will have their own way....none are wrong and all are probably right....you just have to pick your way through the minefield of contradicting advice and choose the method that suits you.....and your wallet, the best
In the end, if you grasp the basics, don't break any of the rules, be patient and continue to learn about your pets, keeping them will be a doddle.
Get some books, learn lots and lots, and ask loads of questions....no matter how silly they seem (the only silly question is the one you don't ask !!!)
Good luck