Imperator: Rome is not an easy game even for advanced players who have had some experience with similar titles. That's why choosing a nation for the first game is not easy.
The game suggests some interesting nations for the first game. Not all are easy to play. For example, Seleucids are a large and strong country, which needs a lot of attention and is threatened by several things. In the first game it will be difficult for you to control all aspects of your state. Therefore it's better to choose a small country, for example Rome, Carthage or Macedonia. These are small countries with good bonuses, geographical position, strong army, etc.
Remember! You can lower the difficulty level to make playing any nation easier. The game will unlock special modifiers, which will make the game easier for you to play and more difficult for AI players.
You can also choose small countries that are either isolated or have a small number of neighbors, such as British island tribes, one of the Crete nations or the southern kingdoms of India. A small territory is easier to control and a small number of neighbors will enable you to be more attentive to your foreign policy.