Their are several things to look out for to begin your journey to success in this field.
A greatly overlooked but defining aspect of a poster is its typography.
Whether your type is wavy and stylistic or rigid and traditional, both can inform your message in a way that is not obtuse to your design flow.
The type of font you choose is secondary to the how it relays it's message, the dreaded 'comic sans' for example has become synonymous with miss-messaging one's work.
Keep it simple and within your design goals, 2 to 3 fonts total are all that's necessary for your overall poster's image, These fonts should be distinguishable enough from each other while keeping a unified design in mind.
Creating balance within your poster is essential to creating a unified, cohesive look. When design elements are not in balance, the viewer may feel uncomfortable looking at your design. In general, there are two different ways to create balance: symmetric and asymmetric. Both have their values to express different designs, pay attention to the work around you and spot how different artist's came to their conclusions.
In a symmetrically balanced layout, similar design elements are aligned in an equal way on either side of the vertical axis. Often, this results in a mirror-image effect. This layout is perfect for a formal or static look. This could include posters dedicated to formal events, art gallery viewings, and informative gatherings.
I fear not the man who has practiced 10,000 kicks once, but I fear the man who has practiced one kick 10,000 times.
No matter who your audience is, colour is a language that is universally appreciated and can communicate a variety of things. It doesn't matter whether you're using colour to demonstrate a specific meaning or just to create a unifying background. Colour can have several different meanings and implications based on the specific culture that you are in.
Colour can also be used to unify one's design, Look at the work around you and see what colour combinations are speaking to a certain design, make them simple make them flow. The world is full of examples of colours working in unison get out there and figure how to apply them to your own work, and see the enhancement in your personal skill level dramatically increase