One of the fun things about an aquarium is decorating it.
The options are endless.
There are as many choices you can make as their are humans on earth!
So how then can we decide?
There are three top level choices to start with.
Natural, artificial and hybrid.
With natural you google the types of fish you have and look at pictures of the fish in their natural environment. (click images)
The same goes for artificial.
Hybrid is up to you where you would select a mixed combination of elements to decorate your aquarium.
The choices are many and self evident as far as size is concerned. Color is a matter of taste or if you go the natural route you duplicate the pictures you have seen of the biotope.
Biotope is a fancy word for natural habitat. It could be a stream or a coral reef.
One way to get ideas is to visit an aquarium shop - the better shops have nicely decorated tanks as well as some if not all of the decorations you will need.
Don’t use wood or rocks picked up in nature. Spend a few dollars on aquarium ready substrate instead.
Keep in mind the following:
Sand or gravel is a pain in the ass. Detritus (fish shit) accumulates under the surface. This can be good or bad - you’ll need to read up on biological filtration to understand why.
One thing I want to mention is that there are numerous excellent books on decorating an aquarium and a library of 2 or 3 books is a worthwhile investment.
Since this is your first aquarium I assume it is freshwater. In fact I recommend freshwater for starters. But if not the same reference materials are available for saltwater tanks as well.
I recommend going natural in appearance and although you can do live plants go artificial first time around – you won’t know the difference and live plants require a lot more expertise.
There is a LOT more I could write - but the most important thing you could do is read a lot. The internet can get very confusing when you solicit opinions and ask questions. For every helpful aquarium owner there will be a different answer to the same question