If you are looking to improve your current fitness level, resistance training is going to be the best thing to do. While cardio work will definitely help you burn more calories and hence lose weight, this weight loss will be a combination of muscle tissue and fat tissue and the result will be a smaller version of your current self.
Resistance training however will really help to reshape your entire body as it will bring your current muscle mass up while decreasing your fat mass. This will help to give your body curves in the right places and make you look thinner yet more defined.
Resistance training is something that you do need to do on a regular basis to see results though; preferably done three to four times a week. This will allow you enough time to fully challenge your muscles but also give your body the rest it needs to grow back stronger.
There are many different set-ups you can use with your program, from full body workouts to split cycles so it may be best to consult a personal trainer to make sure you are doing what matches your needs. If you are highly active in other activities as well, then you will want to tone down the amount of additional strength workouts you do so you do not become overtrained.
Furthermore, you can likely perform your strength workouts so they are more specifically targeted towards the other activities you perform, helping you improve your performance with those as well.
So make sure you don't overlook the importance of this form of exercise when planning out your fitness routine. It truly will change the way your body looks and your capability of functioning effectively in every day life.