Don't worry about the weak thing. There is always a resistance exercise appropriate for a given level of strength. Just make sure you are balancing your push with your pull. If you are doing chest make sure to work your back too. Due to genetics/body type, you might never be huge, but that isn't important. You can be very strong at any given body type.
Quite frankly the key is discipline and persistence. You will simply have to keep at it for years. The reason not everyone is walking around ripped has nothing to do with genetics; it is about habit. You want to make working out an OCD thing for you. Of course, never push so hard that you injure yourself.
USEFUL TIPS:
Weights:
If you want to do weights/resistance every day, a great rule of thumb is to rotate between push and pull. One day do your push stuff (chest and triceps) and then the next day do your pull (biceps, back). Exercises tend to naturally divide between these muscle groups anyway. It is a handy way to make sure your muscles get a day of rest while requiring no notepad to plan. You can even do a once or twice a week day of heavy leg exercises. Being larger muscles, your legs will require more time to recover.
Cardio:
Like Igloodude said, stationary bike is a great place to start. It doesn't matter how good you are relative to other people; simply make sure you are doing about 30min of vigorous exercise per workout. A good rule of thumb for intensity is that you should be uncomfortable, sweating, and unable to talk the whole time. You will get better/faster/stronger, but that feeling should be the same... always pushing your frontier. If you can have a conversation, you aren't trying hard enough and are probably wasting precious time. If possible, use a bike with some sort of stats feedback where you can compare from workout to workout. It makes it less painfully boring, and you can make yourself little goals each day. When you start seeing your Calorie/workout go up over time, it is very rewarding.
Don't bother fiddling with the weight calibration on the cardio machine. In fact, it is best to have it constant even as your actual body weight changes for easy comparison reasons. It is also faster to get going and you can just change resistance manually. You aren't literally worried about the number of Calories you burn; it is just a useful relative measure from workout to workout. The calibration isn't important. This might sound trivial and esoteric, but this is the first thing you will deal with when you go to do cardio.
Use the bloody toeclips. You get an entire upswing motion, working your hams and making your time more efficient. This is not natural to most casual bike riders, so concentrate on making a habit of it. Your pull should be as vigorous as your push. Don't just stand on them, make them nice and snug. Your foot shouldn't be lifting off the pedal. After a while, you won't be able to stand on a clip-less pedal without immediately falling off
Don't use too much resistance There is a tradeoff between resistance and cadence (RPM). This isn't just for biking, but most cardio exercises. It is good to mix up one or the other throughout workouts for variety, but most beginners tend to systematically use too much resistance with too few RPM. Spinning at 90-110 RPM for long stretches is very good. Jumping up to 130+ for short, furious streches is good too. If you have to lower resistance a lot to maintain it, so be it.
General:
Use your time efficiently. Whether weights or cardio, learn to not waste time. You might have hours to waste at the gym when you are young without a girlfriend or family. However, this is a long-term project in your life and your time circumstances will change. You will have to fit workouts in before work/school/dates/driving kids and whatever else life asks of you. Right from the beginning you should be learning to be quick. This is part of why you don't mess around with the settings on the bike... just go and adjust manually. Rest between weight sets, but don't flop around on the bench yacking for 10 minutes. Learn the discipline to start your next set ASAP.
Make certain exercises "interstitial". Resting your triceps between chest and tricep extensions? You might as well like down on the mat and flop around to get your abs worked. Abs are great for this purpose. Waiting for a machine? Lie down and do abs.
If you are with a friend, the two of you can rotate between two different machines. As soon as you are done a chest set, jump on the leg machine your friend was using. He can jump on the chest machine. Keep rotating so that you are always pushing something while resting your muscles and saving time.
Your workout routine will not be sustainable if it takes too long.
