Team fighting with brand is fairly simple. First, i (and you should to) always build rylais crystal sceptor for the extra health and to keep enemies constantly slowed in team fights. Also, i reccomend taking Zhonyas to keep yourself from dying in team fights, because you will be very important to your team.

Team Fighting:

First, in a team fight, try to land a stun on the enemy team's AP carry or AD carry to burst them down and give your team basically a free kill. If you aren't capable of this, engage in fights with your ultimate (R) when the enemy team is bunched together; this will slow their whole team if you have Rylais and will do loads of damage to a majority of the team. When the enemy team is ulted your passive should be activated on many enemies. If you do not know, Brand's passive is if you land 3 abilities on an enemy within a short period of time an AOE explosion effect will go off on the enemy, similar to Zileans bombs (q ability). When you are attempting to stun an enemy make sure to start of using your E to instantly mark your enemy with "Blaze" (your passive) then use your Q instantly after to stun the target. While they are stunned be sure to use your W ability to deal a great amount of damage. Landing these 3 abilities will activate your passive dealing even more damage. Again, here is your 3 ability combo in order to stun the fastest and do the most damage. E, Q, then W. After this combo, if the enemy is around minions or more enemy champions you can use your Ultimate to deal even more damage to your opponent. If you do not know, your ultimate launches a ball of fire, bouncing off of up to 5 enemy champions or minions, which can also activate your passive. This ultimate is great for team fights and very effective. Remember, in team fights try your hardest to focus the AD or AP carry. When killing the AD carry the enemy team no longer has any damage to win fights.

Laning:

In lane you can use your abilities to clear minion waves fast and effectively. Using your W on a group of minions then using your E on a minion that has been marked with Blaze (passive) will spread a ball of fire to each minion near the target, quickly doing damage to multiple minions making pushing out a lane very simple. In a 1v1 situation try your best to avoid any stuns, snares, roots, etc. as brand has no escape and an advantage can turn downhill very quickly. When in range of our e on enemy, use your E then instantly use your Q to stun the enemy. As soon as the enemy is stunned use your W to deal more damage. After this combo you can also ult them, for more damage then ignite to finish it off. Again here is poke combo: E, Q, W All in combo: E, Q, W, R, Ignite
The reason it is better to engage with your E is because when engaging with W you may miss the target and not have that extra W damage when the stun is landed, But you can also use your W then Q to stun the enemy or your Ultimate then Q, as long as they are marked with Blaze when your Q hits them.

Necessary Items: Rylai's Crystal Sceptor, Liandry's Torment, Zhonyas' Hourglass
When building with brand you want to build a more tankier yet heavy AP so you dont get melted in fights. Rylai's is my personal favorite on brand, because all of brands AOE causes enemies to be slowed and the slows can really turn the tides of a team fight. Also, using your E on en enemy slows them, making it easier for you to land your Q for the stun. Always build Zhonyas with brand as this will help in almost any situation. In a teamfight you can use zhonyas to avoid a leona ultimate for example or use it in a 1v1 to simply avoid damage and wait for abilities to go off cooldown.

That is it for my brand guide. If this helped you in any way let me know! I plan on making more guides in the future let me know if you would like a guide for a specific champion. I also will soon be making a youtube channel where will i will post guides, highlights, and more!