I have an idea for a card game. How can I best start testing it?
There are different stages to testing.

You need to make sure the core idea and gameplay is workable.  You can do some of this by playing through a few games with just you playing for each other player.

Then you need to start creating the game.  This will be an iterative process.  You get the game together, and draft some instructions, and get some friends round to play it.

Take many notes!  See how they understand the instructions; make notes of any oddities during play; see if they understand the game in the way you intend.

Make changes, and do this again.  Do this a few times.

Eventually you'll get a game that's ready for beta testing with people who don't know you; maybe as a print and play download. Make sure people know it's a beta test, and make sure they have some good method for reporting problems they have and getting help.

You'll also want to think about asking people to try to break the game; trying to use weird strategies to win or "king making" or rushing or such.