Welcome! I am relatively new as well... Had a reef 15 years ago and worked my way back in with a nano the last few years.
My new cube has been up for only a few weeks and there are already a few things I wish I did a little differently. Hopefully these help a bit
1) Tons of information here to read but be sure to ask some questions that are specific to you. I regret (just financial really). That I didn't seek to fully understand and made a few purchases that will require additional investment.
2) as previously mentioned by other members. Check for the sale threads once you have a good idea of what you want then do a little more research on the items offered. I basically purchased all of my items from members here but admittly paid for a few items inside the deal that I later realized didn't have the same value as I thought.
3) design with care. Every project has some level of compromise just be sure you leave yourself flexibility to adjust/adapt later. For example Not only did I compromise (cost) on a particular item but I doubled down by designing a part of my sump around it. Now I am a bit limited on choices when upgrade time arrives. Leave some space to correct for it later
4) patience. At all levels. I prebuilt/prestaged everything. Redid my plumbing twice etc etc. ordered my last item and it was delayed in shipping. Decided to go ahead and do my final buildout, get the tank cycling.... Now I am working under the tank with everything in the way to get the last items in place. The results are messy and I know I will have to pull out the entire sump at some point to get these items the way I really want them.
5). Brstv. Great how to and overview videos. It can get (more) expensive following along but it will help making some key decisions
6). Better half. Buy in from the significant other. Showed the mrs the leak detection add ons for the apex and it paid for the controller.
7). Keep and eye out for all of my mispurchases showing up in the for sale threads soon so you get a good chuckle of how much I misspent and swear to yourself it won't be you.
Good luck!
George