The chemisorption of methylsilane on the Si(100) surface is studied from first principles. Methylsilane is found to chemisorb dissociatively; during SiC film synthesis using methylsilane the dissociation of the molecule can take place either before or during interaction with the Si(100) surface. In the latter case we suggest a possible dissociation reaction pathway involving the scission of a Si-H bond, while preserving instead the Si-C bond. We investigate the geometry, the energetics, and the vibrational properties of a number of possible configurations following dissociative chemisorption and compare our results with recent experimental data. Finally, the results of a molecular dynamics simulation, aimed at qualitatively describing the process of heating methylsilane on Si(100), are reported.