Sorry to hear you're going through this.
You'll only have a limited amount of time with them, whether it's a free or paid for appointment, so focus on what you want to know. Remember they're a lawyer not a counsellor, so don't focus on why you want to get divorced or your relationship breaking down. Focus on what information you want from the solicitor.
You may find it helpful to ask specific questions about the divorce process, but there is lots of information about this freely available online. If you can, read up on the process and reasons for divorce before you go the appointment and ask specific questions (if you have any) rather than general 'what are my options?' type questions.
I would suggest focusing on discussing what the financial settlement is likely to be (you can ask their opinions on a best case and worse case scenario for you) and what the living arrangements for any children is likely to look like, because these will be specific to you and your circumstances. These will often go hand in hand - if you're planning on having 50/50 care of any children this may affect how finances are settled compared to only having them every other weekend for example.
You could set yourself time limits for how long you want to talk about each topic, to make sure that you cover everything, eg 10 mins talking about reasons for divorce, 20 mins on finances, 20 mins on child care, 10 mins for anything else.
Once you've decided what you want to talk about, think about what information the solicitor will need to have that conversation with you. This may be your income, husbands income, rough value of the house, value of any savings and who's name they're in, both your ages and any pensions. They might want to know how the house was paid for (who provided the deposit, how much the deposit was, and who's been paying the monthly mortgage payments). The solicitor may also want to know how long you've been married, how long you were living together before you got married, and the ages of your children.
I know this must be an upsetting conversation to have with them, but try to stay factual and make the most of the time with them. I hope it goes well.