About brushes, you'll have to take care to use a good quality brush, finest sable ( marten or similar ) like Winsor&Newton's brushes series 7. This is particularly important as it will reflect on the modulation and sharpness of your inking. Better not to save money when buying a brush for inking, believe me! You may want to choose a brush because of its natural smoothness that reflects on the artwork in soft line modulations. The sizes of brushes may vary given the size of your picture ( the larger the picture the larger the brush, in general )... However a good size for general inking ( comics and RPG illustrations ) is N°0 to N°3, because they allow thin and wider lines. My personal choice goes to Winsor&Newton N°0. Keep in mind that I am referring to Winsor&Newton sizes, which are generally a little wider than standard sizes, so a W&N 0 could be like a N°1 of a different brand.
Your brush has to be cleaned after every work! If you plan to use it again in a short time you can just use fresh water, gently taking care not to waste your brush tip, in general it's better to periodically wash it with soap&water: this way you'll not only keep you brush in a good shape but also will be always clean and ink will work better on it. Do not use trementine or other hard solvant because, with ink, you don't need them and they consume the sable of your brush.