ok i don't know what happened to my reply, i had something typed a while ago in reply but never posted i guess :O

first geocities and tripod don't have subdomains for your use.
also

"some letters and numbers.onlinehome.us and it says "this has been registered by one of our customers blah blah.. there's a step i'm missing obviously"

you're only missing the part where you upload your html to replace their default one that is placeholding for you.

most registrars give out free subdomains, this one gives you 50. the reason they only allow 50 instead of unlimited is probably because of DNS limits of their nameservers that they own. whereas with a bigger registrar would probably give you as many as you want, and some hosting gives you it too.

anyway. if you got that site that you explained, just replace your site's html with it.