This question has as many answers as there are styles - but some very general principles for aquascaping in no particular order:
1. Less is More, don't over do it, you aren't trying to cause 'anxiety' in the viewer by them not knowing where to look. Iwagumi is the epitome of 'minimalist' aquascape design, but even if you aren't doing Iwagumi, you want a very clearly defined focal point, and supporting points to 'prop up' the focal.
2. Utilize the Golden Ratio
3. Rules can be broken, but as a beginner STICK to the rules. You can break almost any rule, but you can only really do it successfully if you know how the rules work and 'how to break them' so to speak.
4. Decide what kind of aquascape you want to make and stick to it - do you want it to be impressionistic/emotional (Iwagumi again, is the best example of this, but it's not limited to this), or 'natural' in set up (i.e. emulating something you might see strolling by a stream or the like). These two design styles have vastly different approaches in creation.
5. You CANNOT use wood in an Iwagumi aquascape. Using wood means it is NOT an Iwagumi. You can utilize wood and stone in an aquascape but my advice is for the first aquascape use one or the other - the reason is that stone and driftwood have two entirely different design theories and principles to being utilized, and without some experience with one or the other fusing the two doesn't work out too well, since they will appear awkward.
6. Odd numbers are your friend - regardless of what kind of style, use odd numbers. Odd numbers are both more 'natural' appearing in the literal and figurative sense. Use odd numbers of both hardscape materials AND plants - even in Iwagumi, two plants are a bad idea, use one or three. The reason for this is you absolutely need a transition point plant to act as an accent between plants A and C. This is perhaps one of the biggest common flaws for beginners. Going from say, HC to stem or hair grass plant is an abrupt change that causes tension.
7. It's all in the details - choose a main focal point, and stack on the tiny details, these add a dynamic feel to the aquascape as well as providing subtlety. Tiny details are things such as accent plants, slope variation in substrate, smaller rocks, etc, that add key little details and transitions.
And finally, again, keep it simple (not boring, simple. Big difference), there will be frustrations, you will get algae, you will want to throw your hands up in disgust at the project - don't make it harder than it needs to be for your first attempt. Make it pleasing, and easy for maintenance, and focus on learning and practicing the general rules so that you know how to break them and get even more creative with your designs.
P.S. If you do Iwagumi, please for the love of all that is good in this world, read the article titled "Iwagumi Design" in my signature. Even if you don't do Iwagumi, it can provide you with some good insights.