Where you from manesag? I'm from Venezuela ,I do have some problem with English language too, the most important my friend is the domain of Gmax which is free as we already told you , the primary lesson (No matter if you still don't know to make shapes ) is to export ,
Now there are two diferent ways to export mdl files
1) is by using the FS9-SDK tools using the makemdl9 which would have to be install it first
2) by Using the FSX-SDK tools which is completly diferent that the previous version I'll give you and example
You can crate anything you choose to make a box or a cilinder or a sphere
then EXPORT to FSX or FS9 depend which SDK you have chosen
or pick any shape that Gm,ax allows you to import to then export it to the game
I use one of my folders rename with the name CAREFUL DO NOT USE ANY OF THE DEFAULT MODELS ,use any of the add-on zip files ,like my 727 folder after all you can delete at the end unzip again the 727 and put it back.
Once you can export a model using XtoMdl (fsx-sdk) or makemdl9 (FS9-SDK) both runs in FSX ,You would have win the half of the battle , maybe this would sounds vague ,there are too many things you need to learn , try first by downloading Gmax is free then if you have FSXI will tell you what will be the next step..But you have to calm down and be patience this is not easy as I told you as it looks.
Alex