Investing in an index couldn’t be easier thanks to ETFs, you can either find a World Index tracker like (VWRL) or you can break it down by country based ETFs (like VUKE, VUSA, VJPN etc), it’s mainly down to what you want to invest in and how much time you want to put in rebalancing and researching ETFs in the end.
One thing I would pay attention to is the OCF (ongoing charges figure) of the ETF(s) you want to invest in as they will affect your returns in the long run. You’ll notice I’ve only suggested Vanguard ETFs above, as they tend to have the lowest OCF.
Sidenote: Investing is personal and should fit your goals and risk appetite, so the advice you receive might not fit what you’re trying to personally achieve. That being said, you said you want to focus on index trackers so I’ve linked to Freetrade post explaining ETFs a tad better