Welcome To RS!
There's a lot to this hobby and there are some common mistakes I will make you aware of.
1) Buy quality equipment, this doesn't mean buy the most expensive but find out what others are using and feel is worth the money, this will save you from buying twice also a potential disaster or loss of livestock should a shotty component fail.
2) Research and ask about anything you are unsure of, also double check anything the local fish stores tell you or try to sell you, not saying all shops will give you bad advice or sell you something you don't necessarily need but some will, till you confirm their knowledge and honesty it's better to be on the cautious side.
3) Don't rush the setup and cycle of a new tank, there is no such thing as instant gratification in this hobby.
These are a few problems I see new comers having time and time again, your going to make mistakes no doubt but if you do the research and find out what work and what doesn't you can at least avoid the more common mistakes.
