I'm going out on a limb here. Let's see what the other fellas think of my views.
First rule . . . Don't rush it. You will need to gradually build up your fitness and strength over time. You can supplement your bike training in the gym, but you should bear a few things in mind.
Building muscle (like body-builders do) will negatively affect your cycling. This is because the programs they use are designed to reduce their stamina so that they break down their muscles faster, so that the regenerate muscle mass faster and then grow like crazy. This will 1. make you slow, and 2. reduce your endurance capability. (The point I'm making is don't ask body builders for advice in the Gym.)
Nevertheless, there is no better strength building activity than squats. (It is however also the biggest muscle building activity aswell.) The trick is keep the weight moderate (you should be able to do lots of repetitions like 15 to 20 or more.
I usually like to isolate the different muscle groups in my legs and train them on different days. (I don't know what everybody's views on this are).
I'll do the following for thighs: Squats and leg extensions.
This for hamstrings : Lunges and hamstring curls.
and for calves : Standing calf raises or seated calf raises.
I find this assists me with my power and isn't too detrimental to my endurance. (I do find my speed does suffer a bit though, but I supplement that with intervals)
Hope this helps. Some may view some of this as incorrect or controversial, but you have to find what works for you. Good luck.