Sounds good but could do with a bit more info!
Sounds as if youwant the tank 'emplaced' within a destroyed building that has been fortified with sandbags, am I right?
If so, (and this is a case with any diorama) decide on the story of the scene. Were they waiting in ambush? Are they defending themselves from being overrun? etc. Once you have a story every detail of the diorama will begin to slip together. For example, it helps to know who they are fighting - wether they are on the eastern or western front will dictate the paint and transfer sceme of your Tiger and the uniforms of the men.
On to tips....
Once I have the story sorted I find myself a decent bit of MDF (or other solid, smooth wood) to use as the base. Practice laying out the scene you want to depict and pencil in the positions of tank building etc. on the wood, this should also give you the size and shape the base needs to be. Once your base is cut to size you can begin on your diorama, starting with the floor and buildings, gradually adding debris etc. until your men and tank are ready to be put in.
On another note, be careful with the Tamiya sandbags and brick walls. If they are anything like the 1/35 sets (which im pretty sure they are) they will be very 'uniform' in size and shape and therefore not look very realistic in a diorama. I suggest heating the sandbags when piling them together to get them to 'slump' more realisticly. I also cut bricks out of the tamiya walls (Particularly those 'half' walls) to make them look more realistic (no brick wall will break like that) and to get loose bricks to add around the base as debris from the walls.
Your best bet is to look at the excellent quality dioramas on this site and take note of what you see. As its your first diorama you'll learn a lot of methods, most of which are very personal and you'll perfect yourself! Don't be afraid to experiment and take your time to solve problems as a solution will always arrise! Most of all have fun!
James