Answer by taking a quote from the following article:

Andrew "Rube" Foster (September 17, 1879 - December 9, 1930) was an American baseball player, manager, and executive in the Negro leagues. He was elected to the Baseball Hall of Fame in 1981. Foster, considered by historians to have been perhaps the best African-American pitcher of the first decade of the 1900s, also founded and managed the Chicago American Giants, one of the most successful black baseball teams of the pre-integration era. Most notably, he organized the Negro National League, the first long-lasting professional league for African-American ballplayers, which operated from 1920 to 1931.

Foster was born in Calvert, Texas on September 17, 1879. His father, also named Andrew, was a reverend and elder of the local American Methodist Episcopal Church. Foster started his professional career with the Waco Yellow Jackets, an independent black team, in 1897. Over the next few years he gradually built up a reputation among white and black fans alike, until he was signed by Frank Leland's Chicago Union Giants, a team in the top ranks of black baseball, in 1902. After a slump, he was released, and signed with a white semipro team based in Otsego, Michigan - Bardeen's Otsego Independents. According to Phil Dixon's American Baseball Chronicles: Great Teams, The 1905 Philadelphia Giants, Volume III "In completing the summer of 1902 with Otsego's multi-ethnic team--the only multi-race team with which he would ever regularly perform--Foster is reported to have pitched twelve games. He finished with a documented record of eight wins and four loses along with eighty-two documented strikeouts. Ironically, strikeout totals for five games which he appeared were not recorded. If found the totals would likely show that Foster struck out more than one-hundred batters for Otsego. In the seven games where details exist, Foster average eleven strikeouts per outing." Toward the end of the season he joined the Cuban X-Giants of Philadelphia, perhaps the best team in black baseball. The 1903 season saw Foster establish himself as the X-Giants' pitching star. In a post-season series for the eastern black championship, the X-Giants defeated Sol White's Philadelphia Giants five games to two, with Foster himself winning four games.  According to various accounts, including his own, Foster acquired the nickname "Rube" after defeating star Philadelphia Athletics left-hander Rube Waddell in a postseason exhibition game played sometime between 1902 and 1905. A newspaper story in the Trenton (NJ) Times from July 26, 1904, contains the earliest known example of Foster being referred to as "Rube", indicating that the supposed meeting with Waddell must have taken place earlier than that. Recent research has uncovered a game played on August 2, 1903, in which Foster met and defeated Waddell while the latter was playing under an assumed name for a semi-pro team in New York City.  Now a star, Foster jumped to the Philadelphia Giants for the 1904 season. Legend has it that John McGraw, manager of the New York Giants, hired Foster to teach the young Christy Mathewson the "fadeaway", or screwball, though historians have cast this story in doubt. During the 1904 season, Foster won 20 games against all competition (including two no-hitters) and lost six. In a rematch with Foster's old team, the Cuban X-Giants, he won two games and batted .400 in leading the Philadelphia Giants to the black championship.  In 1905, Foster (by his own account several years later) compiled a fantastic record of 51-4, though recent research has confirmed only a 25-3 record. He led the Giants to another championship series victory, this time over the Brooklyn Royal Giants. The Philadelphia Telegraph wrote that "Foster has never been equalled in a pitcher's box." The following season, the Philadelphia Giants helped form the International League of Independent Professional Ball Players, composed of both all-black and all-white teams in the Philadelphia and Wilmington, Delaware, areas.

When did he start playing baseball?
Foster started his professional career with the Waco Yellow Jackets, an independent black team, in 1897.