Take a look at string.Format...
Here's an easy version of it:
string headerFormat = "\t\tMon\tTue\tWed\tThur\tFri\n";
string lineFormat = "Week {0}\t{1}\t{2}\t{3}\t{4}\t{5}\n";
textbox1.Text = headerFormat;
for (int week = 0; week < 4; week++)
{
textbox1.AppendText(string.Format(lineFormat,
week, toys[week, 0], toys[week, 1], toys[week, 2] toys[week, 3]
toys[week, 4]));
}
Thats the easiest way to do it, you can play around with the format strings for each field to make them line up if they don't.