Okay, very quick (basically commented code): Say what part you don't get.
[01010101]->GDB1   ;this is the tilemap in hexadecimal code.
[01000001]
[01010001]
[01000001]
(Some sprite data to pic1 here)
for(b,0,3)   ;outer loop for drawing the different columns
for(a,0,3)   ;inner loop for drawing the rows
{B*4+A+GDB1}->T  ;gets the tile number minus 1 to T
if T ;checks if a tile should be drawn
T--
pt-on(a*8,b*8,T*8+pic1)   ;draw the tile
end
end
end
That are the basics of the tilemap drawing code.

If you experience any problems please ask.

A for loop repeats until the variable specified reaches the second number (it adds one to the variable each time a loop is ran)
For(variable, start, stop)
so for(a,0,3) will loop from a = 0 to a = 3. The best thing is that you can access the variable inside the loop.

ok.
Wait, I made a stupid mistake
T--
If T
should be
if T
T--


{B*4    ; B is the row number counted from zero, 4 is the width of a row
+A        ; A is the column number (sorry for being wrong there)
{B*4+A  ;gets the tile number counted from upper left.
+GDB1}  ;GDB1 is the pointer to the tilemap.
->T    ;store the number that this returns in T
If T   ; only draw a tile when T is not zero (zero means no sprite)
T--   ;subtract one from T to get the tile number
pt-on(a*8,B*8,  ;draws the sprite at a*8,b*8
T*8   ;multiplies T by 8 since one sprite is 8 bytes
+Pic1  ;Pointer to sprite data

I hope it is clear now


T--    ;subtracts one from T so you do not have to draw

Hmmm...
You can just play around with this code, throw it in a repeat getKey(15) loop, and add Dispgraph:ClrDraw after it. You can for example change numbers in the map (Create some sprites before and put them in Pic1).
Or try to figure out how to draw a 5x4 map instead of a 4x4. The possibilities are endless.
(if you really want I will create a program but not now since I am a bit tired)

Would 01 be
[stuff]->Pic0
[THIS ONE]

So then instead, 10
Would be what i said before?