To make a post on the forum. Make sure you have something to post before making the actual thread.
Then take some pictures. Exterior, interior and stuff like that. People want to be able to see most of your build before downloading it. When uploading pictures I use:
http://photobucket.com . Go there and register. Then you can login and upload the images from your Blockland folder. ( Holding ctrl and then pressing P while in-game takes a screenshot and saves it in your Blockland folder, under screenshots. ) Upload the pictures you want and find the link ending with .jpg or other image file formats. Copy that link. Then go on the the forum and press "Create thread" or something like that.
Then you're ready to make the actual post. Write some information about your build and then post the picture writing
[IMG]the link from earlyer[*/IMG] without the "*"
There you go, I hope it helped. :)