Obviously a plan for where you're going to live and how you will support yourself. You can check whether you will be entitled to any tax credits or benefits online.
Most paperwork you can get copies of through the relevant bodies if you haven't got access. Ie can't find the marriage certificate - you can pay for a certified copy and do all that online.
If you have a joint account, you can get it frozen if he's likely to take all the money out. You would need to do this in person in a branch. It can only be unfrozen by joint agreement and it also stops all direct debits etc, so it's the nuclear option as such.
Open your own current account if you don't have one, have your wages / child benefit / tax credits paid into it. Transfer relevant bills to your name.
If you have assets / mortgage, get legal advice. Some solicitors do a half hour free and will help you find a way forward.
If you can afford it, then legal advice! Get to CAB and find out what you would be entitled to with benefits, tax credits, housing, etc.... If you have DC then get on line and work out how much maintenance you should get. Paperwork, birth certs, marriage cert, passports, account details, savings info, wage info and proof of your DP/DH earnings, pensions info. If you are leaving due to an affair or something similar, any proof you can get would be helpful but not necessary.