Depends on if it has a solid bottom or not. If it has a solid bottom - like a concrete planter with a concrete base - then it is just a large container and needs to be filled with a soil-less container potting mix. And we hope it has some sort of drain holes :-)

If it is only solid sides but a bottom open to the underlying soil/ground, then it is considered just the same as a raised bed and any sort of enriched soil product (garden planting mix, topsoil and compost, etc.) will work fine.

Look for somewhere that sells bulk soil products. That planter will hold about 3 3/4 cubic yards of material and you could go broke trying to fill it with bagged product!

thank you very much for the reply, i am going to start digging out the sand tonight to find out if it has a bottom, i think the bottom is part of the foundation but i could be wrong. I found this bulk soil on home depot, do you think this will work?

http://www.homedepot.com/p/Unbranded-6-cu-yd-Loose-Bulk-Top-Soil-SLTS6/205459977

It could work for the raised bed scenario, although depending on what I was planting, I'd probably add some organic matter (compost). But It's not going to work well if your planter is just a very large rectangular container with a solid bottom. At least not without some significant additions to enhance drainage and increase particle size, like bark fines, granite grit or pumice/perlite. Or even some of the removed sand, provided it is a coarse sand.

And while you are digging out the sand, make sure to check that there are some sort of drainage or weep holes if it has a solid bottom.

Consider filling it halfway with styrofoam peanuts topped with blocks cut to fit the container. You will save on soil and have an easier job moving it around (even rotating it so that whatever you plant grows evenly).