The very nature of our sport encourages growth of the quadriceps and in particular the Vastus Lateralis (the outside part of the thigh) and the Rectus Femorus. Often the Vastus Medialisl is gets under worked and thus weaker than the other 2. This causes a mal tracking of the Patella (Knee Cap), causing inflammation and pain. Long term this can also lead to ligament and cartilage damage. The pain is often in the inside of the knee.
Basically, if you don't already have an injury that needs specific and professional advice, you need to start stretching before and after each session. In addition you need to add some weight training exercise to ensure a balanced training plan which also includes core work. Oh yes I forgot to say, cycling is not great at working the core muscles either!!
I have attached a few popular stretches below for you to try, advice ranges but I suggest holding stretches for 20-30 seconds and do 5 on each muscle. You are not simply looking to warm up and cool down but increase your flexibility which is vital in injury prevention.
There is a huge amount of information on the tinternet about stretching, exercises you can do at home or best of all, if you have time join a gym an get some professional advice.
P.s - if you get to this level my work is done!!!