In the Cards tab, select a battle deck (you can store up to three) and tap the target button to play against a trainer.
The trainers are awful on the whole, but you can at least get a feel for how a new or adjusted deck works, rather than risking losing crowns in proper battles.
Sometimes, you'll find you just can't win. The best thing to do after four or five straight losses is quit the game for at least a few hours.
When you return, perhaps the match-making will be kinder.
Clans are a vital part of Clash Royale. Beyond chat and friendly battles, you can make card requests and donate cards to other players.
The former can be a quick way of improving your deck, while the latter swells your coffers. It's win-win, given that many cards you get from chests won't be useful to you, but might be to someone else. Clash Royale hack
Although you gain XP for upgrading units in your collection, there's no point in doing so for all of them. Instead, prioritise cards you use often, with an aim to get basic cards up to level 8 and above, and rares beyond level 4.
As for spending real money, ignore chests - they're a waste of time. If you want to invest, buy gems, convert them to coins, and grab specific cards you need when they appear in the store.
Clash Royale Cheats