Need some help with S&P 500 Investing

Hi, I'm looking into investing into S&P 500.

I'm quite new to investing in general so just need to ask a few questions basically:

1. Should I invest weekly or monthly?
2. I'd ideally want to buy fractionally and dollar-cost average everytime I invest, e.g. £100 a week/month and buy 1.45643 of the stock for example, wondering if this is possible? (Using freetrade, is there another site which would let me do this?)
3. Is there any difference between all the S&P 500's? (I'm using Freetrade and there are multiple different ones).

And yeah, that's pretty much it. I've tried googling all this but I haven't really found what I needed to know, thanks!