Making a top down shooter in Unity can be done using tools found in the unity asset store and I'll tell you about the tools and different ways to develop a top down shooter.
When we say top down we are referring to the angle that the player will be looking at the game. So in this case the player is looking from the top down onto the game area. With this in mind you can experiment with camera positions to get the best feel for you game.
There are several assets in the unity asset store that come with the things required to build a top down game. These are not free but they take hour and weeks off the process of creating a game.
If you like programming then Top Down Shooter Kit is the one for you because it is a set of C# scripts that can be used to make a top down shooter. This asset requires knowledge of C# and is not intended for non-programmers but if you like to get into the code then this is a good place to start because the code is well documented so you know what things do.
The other kit available for making top down shooters is one called Top Down Shooter Development Kit (TDS-DK) which is a set of scripts and objects that can help you quickly make a top down shooter. TDS-DK comes with some 3D models that can be used to prototype a game and also allows you to make your game using the unity interface to changing settings and manipulate the different aspects of the game so you can quickly get a top down shooter made with no coding. This kit is well documented and has a YouTube video explaining how it works. The developer of this kit is K.Song Tan who has developed several other game kits which are also of a high standard and allow simple game development within Unity.
After you decide on the code engine you will use for your top down game the next challenge to making a good game is the level design. This takes a lot of practice and you can get a head start be using already made objects designed for top down games. There are a lot of them in the asset store which you can purchase. If you want them to look unique then I suggest editing the textures with a graphics editor as this can also be a good way to bring existing objects into the same mood as your game scene.
The other way to make objects for your game is to use standard unity objects like cubes and draw your own textures for them. This is a great way to prototype a game without spending money because you can develop your scenes for free then purchase commercial objects and textured to replace them as your game is starting to require a better visual appearance.
Making use of the unity tools like the terrain editor and applying your own textures to that is also another way you can develop your top down scene without spending money at first. If you do this I recommend keeping all the areas your player can walk on at the same height to avoid confusion.
My aim for this article was to get you started and now you know how to get started I suggest you get into it. Making a top down shooter game can be achieved with the information here so the rest is up to you.