There are really only three things that must be looked after on most cars to assure reliability, as I definite it by 'probability of leaving you on the side of the road.' Here, in order of importance, are the 'majors to major in" as the saying goes.
Tires are the MOST important maintenance item, by far. Every steering, braking, and acceleration action goes through your tires. Rotating tires is nice, but the tire pressure is the most important thing. It must be maintained. Overinflating your tires may cause uneven wear and a harsher ride quality, but underinflation is the more serious risk by far. Always err on the side of higher tire pressure if you have any doubt about the proper value. Far better to lose a litte tire life than to lose your own life from a loss of vehicle control due to catastrophic tire failure.
The cooling system is the next most important system on your vehicle, and next to tires, is the most likely to leave you stranded by the side of the road. Your engine's coolant level and composition must be maintained to work properly. Regularly inspect the hoses and repair any coolant leaks immediately.
The main reason coolant is so likely to leave you stranded is that many people never service the system or inspect it. Everyone knows you should change your oil regularly, though not everyone does. Far fewer people stay on top of the cooling system because modern systems need far less maintenance, often going up to 100k miles with no additional intervention.
This false sense of security can often lead to complete system neglect. That means that leaking pumps or degraded hoses are ignored, because the leaks go unnoticed. But cooling system failure can disable or destroy your engine just as running out of oil can.
Which brings us to oil, the important 'lifeblood' of your engine. It should go without saying that this is essential to engine reliability, but I'm shocked surprised by the number of vehicles that aren't that old that I see blowing the telltale blue smoke of oil combustion, their valve stem seals or piston rings worn beyond hope.
Keep your tires inflated, your coolant level up, and your oil reasonably fresh, and most cars will be reliable enough these days.