Obviously a big subject but the point I would like to make is that you are an expert at law. You can "do" law. You cannot possibly do marketing or planning as well. So, please don't.
OK, you can do some basic outline stuff. And then get the experts in to plan it properly within your parameters.
In my experience, when it comes to business planning you need to know these things: How much profit are you aiming at and when? How many lawyers, clients and admin staff do you need to get to that profit? How much is the above going to cost you?
You can do the outline of these but do get your Financial Director to do the cashflow / budget figures and your marketing consultant to do the marketing plan. Both should be able to save you much, much more than they cost you.
In marketing, do ask for advice. For example if you ask lawyers they will tell you that the Yellow Pages brings them in lots of work but as soon as you measure that source of work you find out it normally does not. In my opinion you need the following basics related to marketing: a website; allow for online payment; take credit cards; use email; measure each lawyers conversion rate, and; measure sources of work.
How many leads per lawyer do you need to get so that you get enough clients to give you the profit you want?
We get 71% of our leads from the Web. If you don't, why not? Cost per client from the Web is about £80. Cost per client from a newspaper advert is about £620. As your teenage daughter might say: "Hellooooowwwwwww"!
Oh, and DO NOT let the "partner who knows a bit about IT" do the website let alone do IT. This is very much a web designer and marketer area. Of course you need to let them know, clearly, what you want to achieve through the website. If you don't want to convert clients and have them pay online, then just get a cheap brochure site for clients who've found you elsewhere to check you out. If you want real clients from the Web it is a much bigger issue needing regular activity after the site is live - if you want it to be found that is!