Welcome :)
My partner made the same mistake walking into a fish store and asking for a 'nemo tank', she was sold whatever they wanted to get rid of. That said you didn't do too badly.
If the guy at your LFS said use tap water, especially when you said it was a marine tank either find a new LFS that specializes in marine or ask someone else there that knows better next time. Tap water shouldn't go into a tank. So many have all made the mistake at one point of trying it once, figuring it was fine as nothing exploded, then having to deal with the consequences later on.
Your LFS is only as good as the advice and service they offer, you can get everything they stock online (cheaper too) but I goto my locals because they provide great advice and care.
There are some really great resources around (massively more than when I first tried marine tanks as a child, restarting this hobby feels like I've started a new degree!) which you should try utilizing and research all you can about what you want to achieve.
For the basics, water and filtration need to be sorted before adding anything. From my own false start I can't stress it enough that you concentrate on getting that right first :)
Fresh water and marine tanks rely on different types of micro fauna to do the brunt of cleaning, they like different habitats. Fresh water relies on nitrifying bacteria and algae types that grow quickly and die quickly so noodles are best with high surface area that 'cleans itself' as the populations grow and die. Marine tanks also rely on nitrifying bacteria but more so on the de-nitrifying bacteria which are microaerophilic (need oxygen, but die if theres too much. They break down the nitrites to nitrates) which like living inside really porous material, like live rock or marinepure blocks/spheres.
So ditch the splintered glass noodles (they'll work but not great) and get some Marinepure spheres or even live rock rubble.
Skimmer isn't a must and everyone has their favourites and debate and which produces the best results etc. Just about anything will do and in the least it'll give you more time between water changes, but none of them will prevent you from having to do them. They're more effective than filter floss and far cheaper in the long run than chemical filtration.
Best advice I read on chemical filtration was to start not using it. Only add it as you need it and monitor your water closely to confirm it's having the desired effect and make sure you remember to replace it regularly! If your nitrates/phosphates/whatever are always spiking and you're going through a bag of Chemipure elite every 2-3 months, fix what's causing the issue instead of covering it up.
Water you have to get right, use RODI for topups, Natural saltwater (NSW) from LFS or yourself (just filter it through a filter sock before adding to tank) just make sure you test it or get a bucket of salt (ASW) and get used to mixing it up. People have success with either one in equal amounts, I just stuck to what was more economical and at 10-15c/lt from the LFS I use that. It's always handy to have salt stored for an emergency though.
Once you've sorted your water source and got a handle on filtration you're going to use, the real fun starts with picking out what you want to end up with, making the scape, then cycling the tank :P