Size matters a lot in acrylic Aquariums and you should take that into consideration when purchasing your first fish tank. While many people are overwhelmed by the thoughts of a big fish tank and may think they are only for beginners the truth is that the bigger your fish tank is the easier they are to maintain. Larger tanks are also healthier for the fish, because of the additional amount of water. Small tanks are subject to rapid changes in temperature and chemical changes that occur because of rapid waste buildup and algae.
Choose as large a tank as you are comfortable with but aim for one that is at least 30 gallons, this is small enough for a beginner while being large enough to ensure that the fish inside of it will be healthy provided you change the tank water when necessary.
Shape is not an important factor to fish health however you should keep in mind how easy your tank will be to clean and whether you will need special tools to get into oddly shaped aquariums or ones that are unusually deep. Make sure that your tank will be easy to maintain if you are a beginner, it is easy to get overwhelmed and get behind on maintenance chores.
Finally you will want to look at whether you want glass or acrylic tanks; there are many advantages to both, so a lot will depend on what you are looking for. If you want a tank that has a different shape you will most likely have to get an acrylic tank. However if shape is not a factor then you may wish to consider glass tanks, as they are cheaper, easier to maintain and last longer. The biggest drawback to a glass tank is that unlike acrylic tanks they are very heavy so if weight is an issue you may wish to consider acrylic for your first tank.