It would be helpful if you could take your ID (passport or driving licence and utility bill)with you as the solicitor won't be able to open a file and start working until that's received.
If you're sure about the divorce you should also take your marriage cert as that will eventually be sent to court with the petition.
I find it really helpful if people have already typed out the following info:
Full names & addresses of parties
" dates of birth of kids
Value of matrimonial home
Estimate of mortgage outstanding
Estimate of assets, ie savings, endowment policies, ISAs, shares etc
Same for any debts, especially joint debts
Each party's income and job title.
Having said that, clients aren't usually that organised so don't worry if you don't feel up to all that.
You should feel that the solicitor quickly gets a grip on your situ, what matters most to you, where you want to be at the end of the case (ie, do you want to stay in the house/sell/buy him out etc).
The solicitor should be empathetic and give you time to cry if you need to, but also professional and make you feel confident that they know what they're doing.
If you get bad vibes from the solicitor, don't go with them and try a different one.