Spark plug replacement

Hi
my car is shaking a little bit. There is noise coming from the driver side but it goes way and come back. The car most shake when I brake it shakes for like 5 seconds then stop and also when I drive over speed 40 it start shaking. So iask few mechanic they suggest me to change the spark plug I dont know I its gonna solve the problem but that is the cheapest part I can start with to see if it solve the problem. Please let me know what you think could be the problem and it will be very helpful if you can post a video how to replace spark plug on c240 2001. Thanks