When can I file my 2017 tax returns?

I am doing freelancing for two different companies and I have several questions. I deliver for them digital files, such as research, PowerPoints or excel files. Do I need to add sales tax to the invoices? When and how do I file taxes? I have never worked before in California and just started working in October last year.

What are the advantages of factoring?
Factoring allows your company to get instant cash, in less than 24 hours. Factoring can be defined as a process when a third party firm, known as a factor, purchases the accounts receivable of a business and gives it cash advance based on those bills (that are unpaid). It has a number of benefits like:
1. Does not require collateral
2. Good use for growth
3. Qualify for more and more funding
Do you agree with this? Are there any more benefits?

Is commission rebate from the buyer's Realtor to the buyer a taxable income? Transaction was done outside of the escrow and 1099-Misc (Box 7) was issued to the buyer following January. 1) is this taxable income for the buyer and 2) if so, is he responsible for SE taxes as well?

Can anyone share some tips for budding entrepreneurs. What are the do's and dont's for them.

Which washer/dryer will qualify for an energy-efficient tax credit?

What are the some available defenses against product liability claims?

Has anyone ever used invoice financing for their business? How was the experience and what benefits did you get?