Blender is free and has a lot of documentation online, but doesn't have built in DTS exporting (although there are plugins you can install). Most active modders use Blender so you can get a lot of support from the community
Milkshape 3D (MS3D) is a paid alternative with built in DTS exporting (+ some animation capabilities not possible through Blender), but there's less support within the community as few people use it.
Alternatively, you could use any other modeling program of your choice and only use Blender to export. PhantOS for example uses Sketchup (the Superior Modeling Tool™) and uses Blender to export as DTS.
If you have absolutely no experience modeling, Blender is probably where you should start. It might seem overwhelming at first, but the UI makes sense after some time. You can also try out Tinkercad (an online modeling program) if you want to try modeling on a simpler-to-learn platform first
You can also get help at the Blockland Content Creator Discord, which you can join through the link below
https://discord.gg/xfPqWKS