Networking 5 computers with 3 different OS systems

I have a very small office (no Network administrator) and I'm trying to network 5 computers with varying degrees of success.

I'm comfortable with computers, but I'm no expert, especially when it comes to networking.

I managed to network an old XP computer with 2 other Windows 10 computers but I'm not even sure how I did it. I just shared the XP directory and I was able to see the folder on the Windows 10 computers. There is another Windows 7 laptop that can see the files, but we haven't tried to network it.

There was another XP computer that was linked but after awhile the OS became corrupted and I ended up formatting it and installed Windows 10 on that computer. That's where I ran into some problems.

Although I can see the other Windows 10 computer from my Windows 10 computer, I can't seem to network the drives by sharing folders. We end up sharing stuff via the Windows XP computer. The computer that I recently installed Windows 10 on can see the internet but I can't seem to network with it at all. It doesn't see any other computer on the network. When I first installed it, I was able to hook it up with the network printer but after I tried to network with it, I could no longer see the printer either.

I've tried pinging the new Win 10 computer and I've been successful but the new computer cannot ping anyone back. I've tried disabling firewalls, but that doesn't seem to do anything. I can't even see the other computers when I use the arp -a command, nor can I see the printer (although I could originally).

I have tried uninstalling the network card and then re-installed it, but nothing. I've run the network diagnostics program. No luck.

Also, the new computer cannot see the XP shared drive although all the other computers in the network can.

Any suggestions?

Agh. After working on this all day, I noticed that the computer was plugged directly into the modem not the router, so of course it was on the wrong network.