The core theme of BitLife is to simulate and play any mainstream career path of your choice. On that note, one of the most popular life that people want to replicate is to be a professional soccer player. To do this, you must start practicing soccer from a young age and follow a strict training regime.
Before practicing soccer or joining any club, select a character with a special trait/talent in sports while creating life. Otherwise, reroll till you find such a character or customize such a life using the God Mode in case you have the DLC.
After creating your character, keep yourself athletic and brisk from a young age by visiting Gym, going for a walk, and eating healthy once the features are available from the Mind & Body section under the activities tab. Repeat all three prescribed activities like a yearly ritual till you become a professional player.
Now, as soon as you reach middle school, give tryouts for the soccer club through the extracurricular section from the activities tab and remain a part of it till you reach high school. Put in extra effort by using the Work Harder option to improve each year as a player in the junior division. After reaching high school, reapply for the soccer team and repeat the same procedure you followed in your junior years. Also, try to become the captain of your high school team by using the Work Harder option repeatedly, as that will play a major role in helping your journey toward becoming a professional player.
After graduating high school, you should start receiving offers from various clubs. Accept anything that comes your way. If not, request a tryout in any low to mid-tier clubs in any league. Check the league's standings to identify the bottom teams in the table. Apply to them and hope for the best. With time and effort, you will be selected for one or the other professional team, achieving your dream as a professional player.