what are some way i could make a tank(sherman M4A1) look realy. like what stuff could i add on or make

You could add lots of "crew stowage" ie. tarps, bedrolls, rations boxes, spare ammo cans, backpacks, etc.. You could make an antenna out of stretched sprue too. Also, you could add spare tracks, and fuel cans.

And to see where all this stuff goes, you have to check pictures !

If you have the Italeri M4A1 kit, a lot of modelers have been know to "paint" the upper hull before assembly with liquid cement. This is called "strippling" and the liquid cement causes the smooth plastic to gain a more cast metal texture. Be sure not to do this over the various fittings, hatches and panels. Just do it on the cast hull portion. Testors liquid cement with the black bristle brush attached to the lid works best for this. Do one section at a time and handle carefully.

Real easy to do and makes the the hull look a little more "real" once the final coat of paint is applied.

You could also add some mud or snow, dust and rust -- but before you do add them, think of a particular scenario ,setting or environement where your tank would have experienced...