Competitive PlayWhile you can definitely have a Fennekin on your team, it's not really a good idea to play competitively with one. Its stats will always be limited because it was unable to get that boost from evolution. This is a totally different story if you're playing in the Little Cup class. Little Cup rules state that only unevolved Pokemon can compete, so Fennekin is not necessarily outclassed. I like to use my trusty friend Smogon to detail good movesets for competition. For Fennekin, it recommends the Magician set. The requirements are as follows:
What's really going to set Fennekin apart here is its ability, which allows it to steal the opponent's item. If that item turns out to be a berry, or a stat increasing item then this really can make a difference. But the most fun thing here is that the Power Herb item removes the charge time for one move. So you basically have a Fennekin with a high special attack, that has Solar Beam which requires no charge to use. The only downside is that the Power Herb can only be used once. So you have to make a decision and hope for the best. Fennekin isn't the best Little Cup Fire type, but it does have those unique characteristics. To read up about Magician a little bit more, check out the Smogon Fennekin page.
I terms of just regular gameplay, you really don't need to worry about any of this. X & Y and the generations that followed it were particularly easy in my opinion. As long as you've got a good Fire + Psychic move in for Fennekin, then using it will be fine. But if you want to go competitive outside of the Little Cup, you are definitely going to want to evolve your Fennekin up to a Delphox. You wouldn't want your adorable little fox getting tramped, would you? Once you get your Delphox, you'll once again want to check with Smogon to determine the best movesets. But honestly the first thing you'll want to do is breed a Fennekin with a good nature, and good stats. Perfect IV's are pretty much a requirement for competitive Pokemon. While perfect IV's all the way around are always preferred, with Fennekin/Delphox you can ignore the attack stat in favor of special attack.