1) Keep it simple. Don't try to use rainbow colours for your site. Stick with one (or more if they go together) colour and make a website out of it. I tend to stay with grey, but it's up to you. Even purple works if you do it right. For beginners though, stick with white first.
2) Constant content update. I can't stress this enough. If you only update your website once a month or have nothing useful on it then no-one will come and visit it. Those who do are likely not to come back again. Easiest way to generate recurring traffic is to make a forum of sort, but this isn't possible on GeoCities, so you'll have to go full force on content. Blog and such might work.
3) No counters! Make a counter so you can track people's numbers, but don't put it on your website. It only makes your website n00bish.
4) No MIDI background music. Ugh. No one wants to hear your music when they're listening to their favourite music. Even if yours is better.
5) Learn CSS. It's the best layout management language out there. It's not hard at all either.
There are probably more, and even more, but that's all off top of my head. Good luck
BTW, my first website was plain text. Black and white. Nothing else. Remember that everyone starts with a bad website