What kind of mistakes can a new meditator in his/her early 20's avoid?

I would say that a good thing is to read up on the basic method of whatever meditation practice one wants to learn and practice. By doing this one is making sure that one is really practicising e.g. Samatha Meditation and not Vipassana Meditation.

One could find a teacher in real life or tune into the enormous amount of meditation videos that are available on the internet. One could preferable find a small number of teachers/monks that teach the meditation method one wish to practice.

One could also sign up for a retreat and learn about the meditation-technique in this way. In retreats such as the Goenka 10-day retreat there are also teachers that one can have interviews with if one needs help or assesment of one's practice.

Next there is the practice. Experience is ones friend. Errors will be made. Thats just how it is since its a learning experience and one will become better and better with time. Especially Samatha Meditation is an ability/skill that will improve over time with consistent practice.

I already mentioned it -> Consistency in practice is a key point in ones practice - especially in the beginning where the terratory is new. One does not have to meditate for hours everyday even 5-10 minutes is enough if done consistently. Then one can gradually extend ones sitting time and more importantly begin to carry the meditation over into daily life, i.e. mindfulness in daily activities such as walking, brushing teeth, taking shower etc.

Lastly i want to mention The Five Hindrances which are obstacles to meditative progress. One can read more about the hindrances here and how to overcome them. It helps a lot to know what obstacles one will be dealing with in meditation, especially in Samatha Meditation where concentration is the key point one is developing. When one knows how to deal with e.g. sensual desires, ill-will or worries then one will be much better equipped to practice. Its similar to a soldier receiving combat training before going to war.

You are doing as you should. However, you must avoid negative acts (whatever they may be), at all costs, to all living things.

Make peace with your enemies and allow karma to take it's course. Practice, meditate, and always remain positive. The universe will unfold in your favor as it should.

And you shall know when you have achieved enlightenment. But not without great sacrifice and faith...

When you meditate you get either positive, negative or neutral experiences. Early on in can be very negative and bizarre experiences to a novice meditator. Whatever the case the main and most grave mistake any meditator can make is to loose awareness and equanimity in facing the experiences.

If you maintain awareness and equanimity in facing all experiences then nothing can go wrong in your meditation.

Initially you get mainly painful sensations, then it becomes mostly blissful sensations and then it starts turning to neutral sensations. In the latter case sometimes it might be difficult to keep your Equanimity. In which case you have to try to balance this with Concentration as this it one of the Hindrances (Restlessness) which has come about. The factors to balance the Hindrances are the 5 Powers.

Choose the middle between all extreme contrasts.

If you are going too fast, go slower.

In your meditation you might choose a kind of meditation. Then you would try to keep yourself on doing only that one thing you have choosen. You would be disturbed by thoughts or feelings and might not keep yourself on one point. This is OK. This is meditation: if you come back from things which disturb you, back to point you have choosen. - Many people might think they do not meditate if they can not keep themself on one point - that is wrong.

I think you allready know you need patience, but also there might be a time where a change can help.