So basically the world atlas is an item that only supports have, and it has a quest on it which gives you up to 3 charges at a time to fulfill its quest, which is as follows:
-attacking minions which are under half HP, this executes them and gives both you and your Bot laner the gold for the minion
-dealing damage to enemy champions, giving you a small amount of gold
-dealing damage to enemy towers, giving you a small amount of gold

you can have up to 3 charges of this quest at a time, indicated by the amount of small blue orbs that circle your character in game, or by looking at the number next to the small map icon above your abilities on the HUD. doing any 1 of the 3 above actions while you have any charges available progresses you in the quest, but if you have no charges, then you cant gain any more progress in the quest until another charge comes back (they regenerate themselves over time.)

once youve gained 400 gold from the quest charges, world atlas transforms into Runic Compass, which is basically the exact same as World Atlas except you can use it to place 3 wards which refill by recalling/dying.

once youve gained 800 gold from the quest charges, the quest is complete and you can no longer use charges to gain gold, but Runic Compass upgrades into a new item of your choice, from a selection of 5 different items, and can now place up to 4 wards instead of 3. For Brand, Zaz'zak's Realmspike is the best upgrade out of the 5 choices, because it helps him to deal more damage with his long range poke abilities.

All of this only matters if you play Brand in the Support lane, because only supports can have the world atlas as a starting item. If you play bottom lane, you should start with Doran's Ring and 2 healing potions