This depends on the size of your operation and your goal. If you intend to remain small, perhaps as a substitute for working for someone else, a sole trader might be the best option for you. However, remember sole traders have certain limitations such as being charged at marginal tax rates which are progressive with corresponding levels of income. This on the good side means that you can take advantage of the tax-free threshold of $18,200 as well.
A company, on the other hand, pays 27.5% from the first dollar of profit made. A company has limited liabilities though and may be apt if you are planning on scaling your business higher.
A trust, especially if discretionary in nature on the other side can allow you the flexibility of distributing income among the beneficiaries and save overall tax liability for the group.
This is part of business structuring and we advise you to take professional assistance for setting the structure right.
Whatever be the structure, if you want to trade, you need an Australian Business Number (ABN). This is in addition to a Tax File Number (TFN) and/ or an Australian Company Number (ACN). Depending on whether you employ businesses or have an annual turn over in excess of $75,000, you may also need a Pay As You Go Withholding (PAYGW) or Goods & Services Tax (GST) registrations. There might be other registrations and licensing necessary as well; thus we advise you to take professional help.