Tyres
The tyres are the only part of the vehicle touching the road. Everything the brakes and steering can do is limited by them.
Check pressure regularly and when the tyres are cold. Under-inflated tyres overheat, wear at the edges and use more fuel; over-inflated ones grip less and wear in the middle.
Check the tread depth and look for cuts, bulges and embedded stones. A bulge in a sidewall means the tyre can burst and must be changed, not watched.
Carry a usable spare and the tools to fit it. A spare you have never checked is not a spare.