"A good roommate" is a very relative term. What constitutes a "good roommate" for one person may well be a living nightmare for someone else. 
Here is a list of very simple rules to follow to make sure your flat share goes well.

Be respectful 

If your flatmate is sick in bed with the flu, it's probably not the most appropriate time to listen to your favourite album at full volume, or to vacuum every corner of the flat. You don't have to be perfect all the time, just have a little common sense every now and then.

Pay your fair share, without delay

As a flatmate, you must pay your share of the rent and household bills. The bills must be divided equally between the people living there. Pay your share and pay on time. No excuses.

Replace what you consume

It is impossible to live in a house without using or consuming other people's property. It is likely that at some point you will accidentally finish someone else's milk or use your flatmate's hairdryer. If you happen to eat, lose or break something that does not belong to you, replace it every time and as soon as possible.

Clean up after yourself

It's all very well to leave a pile of dirty dishes in the sink if you live alone, but if you live with other people, it's just plain rude.

Don't leave expired products in the fridge, take out the bins when necessary, and clean up after cooking. Simple.

Be pleasant

Everyone has bad days from time to time, but if you make your housemates suffer your bad mood, life will soon become very unpleasant for everyone.

Choose your battles

If you live with other people, you will need to compromise. No one likes used tea bags cluttering up the sink, but it's not the end of the world if your roommate forgets something on the counter sometimes either. Pick your battles.

Use your skills

We're all good at something - and it's often things we enjoy doing. If cooking is your thing, consider making the meal more often than the other residents, in exchange for them doing more cleaning for example. Whatever your skill set, share it.

Discuss often

Most disagreements start (and continue) because of a lack of communication. If there are problems, you need to discuss them. It's much less awkward to politely ask your roommate to stop using your hair straightener than it is to boil it gently for months. Keep the lines of communication open.