After setting your fitness goal you are on a search for the right workouts.
There are a variety of workouts that you can do that encompass the main aspects of cardiovascular training, strength training and lastly, flexibility. How you schedule these workouts into your week is going to make a noticeable amount of difference in the results you see.
If your main goal is to gain muscle mass, you are going to want to perform more strength training session than cardio sessions. You would be advised to shoot for around 3-5 strength days and 1-2 cardio days.
It is still important to do some cardio to keep your heart healthy but if you do too much it is going to sacrifice your strength gains.
If your goal is cardiovascular fitness however, then obviously aerobic sessions are going to be a priority. You should aim to include at least one longer day in your week where you go for 45-60 minutes at minimum. Then supplement that with 2-3 shorter sessions of 20-30 minutes and possibly one or two interval sessions where you will go at a harder pace for 30 seconds to a minute and then take a 1-2 minute active rest break. Repeat that process 4-8 times in total.
Finally, with regards to flexibility, you are best to do this on a daily basis. Flexibility is something that takes a great deal of practice to become better at it but luckily does not involve a large time commitment. Spend a good 10-15 minutes a day, either after your exercise session or while you are watching TV on stretching and you should see noticeable improvements within a few months.
Taking the time to plan a proper program, rather than just going into the gym and doing whatever you feel like, will place you at a much higher likelihood of achieve fitness success.