Now, my TH level is 7. When is the best to move to TH level 8?

The best time to move from town hall 7 to town hall 8 is when you have already maxed out your army and spells as well as the defensive structures.
In this way, you can focus on new building and upgrades made available by town hall 8.
In upgrading structures, always give top priority to offensive buildings such as the laboratory and the army camps.
Upgrading them first will give you an advantage during wars.
Upgrading the defensive buildings on the other hand, is also important when it comes to defending your village from enemy attacks and preventing other players to take away your resources.