If a question says in triangle PQR, How do you which point is P? Or Q or R? Also it says side p is 18 cm and side r is 20cm. Where do you put the p and r? Which side?

It doesn't really matter, so long as your sides match your lengths. P, Q and R are just pronumerals, not actual numbers. The wording is a bit dumb though, usually, it will say something like side PR or side PQ.

If a question says in triangle PQR, How do you which point is P? Or Q or R? Also it says side p is 18 cm and side r is 20cm. Where do you put the p and r? Which side?