A great way to alter your routine is add stuff to it, or add additional weight to it. Kettlebells are perfect for that, they are balls of iron with a handle attached to it. Having the handle makes the kettlebell easy to use in virtually any exercise that you do. Over time, muscles that are not normally targeted will become stronger giving you a more natural look.
If you were doing squats in your routine than you can add in kettlebells quite easily. First off, grab a kettlebell by the handle with both hands and hold it behind your back. Keeping your back straight, continue doing squats as normal. You will notice that you will tire out faster which means the extra weight is doing its job, holding it behind your back also helps in your form as you are doing the exercise.
When it comes to the size of the kettlebell there are a few different ones. I'd pick one of the smaller ones if you're just starting up though. By picking a larger kettlebell you can get hurt pretty easily because you are not used to it. When you include a kettlebell into your routine you are going to target muscles that you did not use before, and you'll feel them after your workout, even the muscles you did not know you had a bit tender. Once you get used to it than you can advance yourself up to the 20 kg or even the 24 kg model. There are a few larger ones out there, just get what you are comfortable with. Once you are comfortable with a few of the exercises you can change things up and include kettlebells in other routines.
You need a strong core in order to do anything with the rest of the body. You can do snatches with kettlebells to increase your core strength quite easily. Hold the kettlebell with both hands between your legs in the squatting position, next use your body to lift yourself, bringing the kettlbell up at the same time. Keep your arms locked and swing the kettebell up over your head. This exercise is a replica of what is needed for most sports making this exercise pretty good to have in your routine if you're an athlete. Another good thing about using kettlebells is that your grip will become stronger from manhandling pieces of steel all day which is a added bonus.
Effective training is a must as an athlete. Kettlebells provide resistance where you would normally not have resistance. While playing sports you will find that you will encounter resistance in similar ways. Kettlebells are also great for the fact that they are cheaper than home gyms and they are quite durable.