There's a big difference between legitimate supplements and metabolic drugs. A lot of people have totally unrealistic expectations when it comes to nutritional supplements. You can eat a ton of protein and you won't turn into an Arnold unless you follow a strict workout plan. But if you're training very hard then your nutritional requirements are going to be much higher than those of an average Joe. Hence taking proper supplements in addition to a sound diet will allow you to train harder and develop a stronger physique. On the other hand, if you train hard but your diet is inadequate your body will break down, you will end up sore and your performance will suffer.

As others have pointed out, you will not gain any mass if all you do is cardio (cycling). It takes strength exercises, either weights or bodyweight, to build up mass. Doing excessive cardio will actually reduce your mass, fat as well as muscle. That's the reason why marathon runners are so thin.

If your workouts involve a lot of cycling you should definitely eat a lot of protein to help rebuild your muscle fibers. But if you want to gain functional mass, reduce aerobic work (slow, long distance) and do shorter but more intense anaerobic workouts (sprints, fartlek, intervals, Tabata) maybe 3x a week. Combine with strength workouts and you will notice a considerable improvement in strength as well as stamina. Anaerobic work in fact improves your aerobic capacity as well but not the other way around. This supposes you are in a pretty good shape already and follow a good diet. Taking supplements such as protein, a good weight gainer (complex carbs, protein), maybe multivitamins if necessary might help a lot, especially if you are a naturally lightly built person with a fast metabolysm.

Creatine is no wonder supplement. It improves your short term anaerobic endurance somewhat, giving you a slight boost for a few seconds of intense exertion. When boxing I could notice a small improvement in performance as boxing is about short bursts of very strenuous activity. Though in the long run I think it's a fairly worthless supplement for boxers. Weight lifters may benefit more from creatine, especially on lower rep sets. If you decide to take creatine make sure to do research first and only buy powder. Liquid creatine is unstable and for the most part doesn't work. The real advantage of creatine for building mass is that it allows you to lift heavier weights, hence build more muscle. As a side effect it also temporarily makes your muscles look bigger because it charges the muscle cells with water so to speak. But once you stop taking creatine (usual creatine cycle takes about 8 weeks) then these superficial gains will disappear with water loss. Which isn't really important thought as it's only cosmetic and does not in itself produce any functional gains.

Anyway, you will not become a bodybuilder unless you train and eat like one. Understand what the supplements are all about and use them if you have a valid reason to. You also need to understand the difference between functional and nonfunctional mass. Bodybuilding produces a lot of hypertrophy but not much in the way of real strength gains. If you are concerned about functional muscle (eg. performance sports, cycling, weight lifting, martial arts) do not obsess with increasing your mass but rather strive to improve your strength and stamina while following a good wholesome diet. Good physique will follow.