myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login=sl AND password=sp";

myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login="+sl+" AND password="+sp;

myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login=@sl AND password=@sp

myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login="""+sl+""" AND password="""+sp+""""

myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login=? AND password=?";
// параметры подставляются по порядку, именя можно задать любые
myOleDbCommand.Parameters.AddWithValue("Login", sl);
myOleDbCommand.Parameters.AddWithValue("Password", sp);

myOleDbCommand.CommandText = "SELECT Login FROM People WHERE Login=@Login AND password=@Password";
// параметры подставляются по имени, старый Access такое может не переварить
myOleDbCommand.Parameters.AddWithValue("@Login", sl);
myOleDbCommand.Parameters.AddWithValue("@Password", sp);