My TH became level 7, and I am able to build Dark Barracks and Hidden Tesla. There are many kinds of building I can build. Which building should I build or upgrade as priority?

Cannons, walls, and regular barracks

BARBARIAN KING ALTAR OR HIDDEN TESLA

Upon reaching town hall 7, your priority should be upgrading your barracks, spell factory and army camps.
Upgrading your barracks from level 8 to level 9 is very important so that you can start training dragons in them.
Dragons are the most commonly used troop for battles in this town hall level.
Also upgrade your laboratory so that you can upgrade your dragons to level 2 after the construction is done.
Next is to upgrade your spell factory so that you can bring more spells to war with you.
Upgrading your army camps are also important to increase the number of troops that you can house in them and bring to war later.
Building a good defense is also important so make sure to mix in building of new defensive building such as the hidden tesla.
A good defense is important to prevent other players from getting too much of your resources and trophies during raids.
Also upgrade your existing defensive structures such as the mortar, wizard tower, cannons and archer tower.
However, do not upgrade defensive structures at the same time because it will significantly decrease your defense.
Defensive building ongoing upgrades cannot attack enemy troops.
It is ideal therefore to alternately build or upgrade offensive and defensive structures in proportion with one another.