Upgrading is the most important thing in Clash of Clans. It keeps your base up to date and can protect the base from intruders. It also helps you produce your resources faster.
Defenses, traps, and walls should always be upgraded first. This includes the Clan Castle. This way your base will be well built and protected, able to clear any intruders from getting to your resources and Town Hall.
Army buildings are to be upgraded second. This way you can take revenge with an even powerful army. This will also help with farming to accumulate more resources. Any heroes should be upgraded first or last, not in the middle.
Any resource related buildings should be upgraded third. Any storages should be upgraded the first of all resource buildings. Then collectors, mines, and drills are next. If possible, get a few builders.
Town Halls ae always upgraded last. You have to max all of your base. If you are Town Hall 10 and have all level 1 defences, then all your resources can be easily taken, even if you had an Inferno Tower. If you have a Town Hall 7 and your base is max Town Hall 6, you should be okay.
Farming is one of the most important things in Clash of Clans. By farming, you can increase resource income and upgrade more of your buildings faster, especially walls. Walls take most of your time to upgrade, even though theydon't take any time to upgrade at all. On this wiki, you can find lots of helpful strategies to help you farm anything from gold/elixir to dark elixir.
Joining clans are very important. It is the only reason this game is called Clash of Clans. Making a clan can take a long time to become popular so it's best to join another clan. The best clans to join are family, friends, or another powerful clan. Join clans that are the way you want it. If you don't like clan wars, then join a clan that doesn't have war a lot. Clans that donate big and powerful troops. Active clans. Anything like that. I am in a clan with family, so I don't care about my friends.