Tyre wear is inevitable, but the wrong kind of tyre wear can be very dangerous and costly. The signs can sometimes be obvious and sometimes not so. They can appear as unusual noises when you are driving or the feeling of the car pulling to the side when driving in a straight line.
The minimum legal tread depth is 1.5mm of tread so if any part of the tyre is below this they will need to be replaced. Additionally if steel wire is visible on any part of the tyre or bubbling on the side wall they will also need to be replaced.
Easy ways to check your tread depth is with a 20c new zealand coin and if the number 20 is visible you have less than 2mm tread left. Most tyres also have bars in between the tread, if the tread is flush with the bars it is time to replace your tyre.
It is best to always make sure your alignment is done correctly when fitting new tyres. If you feel your car pulling to one side as you are driving in a straight line or making unusual vibrations, it can be signs of bad tyre wear. If any of these signs appear It is best to make sure you take your car in to get the alignment checked as soon as possible to prevent it becoming a much larger issue.
At the extreme end, these wear patterns can cause tyre blow outs which can be dangerous and cause crashes. Always follow the manufacturer directions on correct tyre pressure which can usually be found in the passenger or driver door sill (also make sure the corresponding tyre size is the same as the one found on the door sill).
Below is a diagram on what numbers and letters correspond to on your tyres. This is useful for finding your correct tyre pressure and for when you need to change tyres. One thing to remember about tyres is that they are the only thing on your car touching the ground, so you should never skimp on quality.