Company type - First of all, do you have/need your own Company? (e.g. I set up a Ltd co, as it limits my personal liability - I won't lose my home if a client decides they want to sue me for a real or imagined error). If you're based in the UK, there's loads of clear, relevant guidance on .GOV.UK - www.gov.uk/set-up-business
Remember there are specific legal duties that come with certain types of set up, like a Limited Co. Look into all of this before deciding, then educate yourself on what you need to do and when (e.g. filing confirmation statements etc).
Insurance - Next, do you have insurance and what type will you need? e.g. Professional Indemnity Insurance would be a given for someone in your type of role (I use Hiscox, but there are lots out there, and you should get quotes and compare different terms before deciding on the right type and level of insurance, as well as provider for you). Will you have clients visiting you at your own home/premises? If so, Personal Liability Insurance may also be something for you to look into. Also think about the items you need to do your job - will you need insurance to cover the cost of loss or damage to e.g. your laptop/mobile.
Bank account - how will clients pay you, and in what currency? The type of business you set up (e.g. Limited Company) will have an impact on your banking needs. Limited Companies really need their own bank account (i.e. not your personal account). Consider high street banks which offer free banking for startups (e.g. HSBC used to offer about 12-18 months free business banking for new companies). After the free period, business banking gets expensive - £8 a month, possibly with extra fees for depositing cash/cheques. If you get paid in a foreign currency, find out what traditional banks will charge, then look into challenger banks and alternatives like Wise.
Fees - really think carefully about your hourly rate, and remember your rate needs to cover the cost of your NI, tax, insurance etc., as well as the skills and experience you bring to the job. I read a month or so a go that a freelancer in the UK should be charging at least £25/hr in order to actually take home minimum wage - don't take this as gospel, I haven't actually worked out the sums, but it is food for thought. Personally, I prefer to charge a fixed fee for a project, but I always base this on an hourly rate anyway, so you need to know your figure.
Contract - draft a contract and get it checked by a solicitor. Seriously, don't skimp on this and don't work with a client who won't sign it. They're exactly the type who will take your work and not pay you.
Invoicing - get this right from the get go. Do not offer your clients credit which is, in effect, what big corporates who can afford to offer 30 day payment terms do. The options I would suggest considering are:
-Payment of a proportion of the fees upfront (I charge 50%)
-Payment of invoice upon receipt or within 7 days of the date of the invoice
Make sure every invoice includes what is legally required, as well as your Ts&Cs, especially things like your payment due date, when queries must be raised by and your right to charge interest and a late payment fee for any late payments.
www.gov.uk/invoicing-and-taking-payment-from-customers/invoices-what-they-must-include
Scope of Work - have a briefing meeting for every piece of work your client asks you to do. Follow up with a written scope of work which the client must sign and return for you (along with whatever fee you have agreed, if you go down the part payment in advance route) before work can commence. (Even if it is something seemingly simple like, 'find us a few free images to put on the cover of this brochure'. Get clarity, in writing, over exactly what is required. Taking the time to do this upfront will save you stress, heartache and money later on.)
Kit - what hardware/software will you need to do your job? A monthly Adobe licence is actually pretty expensive, for example - you need to factor this into your fees. Do you need to buy a new laptop/phone/get a dedicated landline/fast internet etc.
IR35 - look into whether you are inside or outside IR35 and perhaps rejig your hourly rate accordingly. This is important for your client to know as well, as both of you must ensure you're paying the correct tax.
www.gov.uk/guidance/check-employment-status-for-tax
Sickness / Holiday - what will you do for an income if you are sick or want a holiday? Consider putting in your contract the option for you to substitute another person to do the work (this is key from an IR35 point of view as well). How big is your network? If you do need to hire another freelance to cover work for you, how will you find someone skilled/trustworthy?
Tax/Payroll/Accounting - budget for an accountant. It's worth it.
In the UK, you can pay yourself approx.£400 per month without paying tax or NI. Anything above that, has to go via payroll and payroll now has to be automated (i.e. you, or your accountant, must use a payroll system which links directly to HMRC). This really is NOT my area of expertise, but that's how I recall my accountant explaining it to me. Pay someone qualified to tell you the current, correct rules and how they apply to your situation.
I'm sure there's loads more, but those are the main ones I can think of!