After the MThd chunk, you should find an MTrk chunk, as this is the only other currently defined chunk. (If you find some other chunk ID, it must be proprietary to some other program, so skip it by ignoring the following data bytes indicated by the chunk's Length).

An MTrk chunk contains all of the midi data (with timing bytes), plus optional non-midi data for one track. Obviously, you should encounter as many MTrk chunks in the file as the MThd chunk's NumTracks field indicated.

The MTrk header begins with the ID of MTrk, followed by the Length (ie, number of data bytes for this track). The Length will likely be different for each track. (After all, a track containing the violin part for a Bach concerto will likely contain more data than a track containing a simple 2 bar drum beat).

Here's what an MTrk chunk looks like if you defined it in C: 

struct MTRK_CHUNK
{
   /* Here's the 8 byte header that all chunks must have */
   char           ID[4];   /* This will be 'M','T','r','k' */
   unsigned long  Length;  /* This will be the actual size of Data[] */

   /* Here are the data bytes */
   unsigned char  Data[];  /* Its actual size is Data[Length] */
};