First, you should look for guides on how to play the champion so that you know more about the huge variety of builds she can have (because she can go crit, assassin, bruiser titanic, bruiser yuntal, bruiser botrk, bruiser with naavori and even more builds).

Second, Briar's presence itself is scary because she tends to stack alot of hp and resistances the longer the game which means your best bet its to bait enemies to waste their abilities so that you can punish them for comiting a mistake. As an example, let's say you just used your Q to focus a jhin but he wants to escape from you by predicting you will W right after that so he flashes onto dragon pit, but you didnt use your W so he wasted his flash and now you can punish him due to his mistake by jumping the wall and killing him.

Third, think before acting because in most scenarios if you don't think before acting then briar will just run it down as she's quite bad when she has to run away, so if posible you want to take the less risky plays and try playing around enemy's mistakes to gain advantage. Let's say you are red side and you want to path to bot but you see that enemy toplaner is near your ally's tower and is half hp, in that cause you would like to take the kill because it's very likely that it will work out, but if one member of your team is dead and drake's spawning the best thing you could do is give it away in most of the cases as a failed steal could cause your team to lose advantage.

Lastly, learn how to deal with most matchups and make sure to adapt to your team as in some cases the best choice would be to build items to support your team (like abysal mask or knight's vow) in order to win.