Sage can be pretty daunting at first, a lot of the terminology can be confusing. I find a lot of the transactions repeat so after a while it gets easier. Generally most stuff is done in the bank module. While in there you can pay bills or use the supplier/customer part in bank to record payments.
I was that overwhelmed the first year I had the software I never used it

once you get into it though your life becomes a lot easier. I could have just passed my stuff over to the accountant but that would obviously come at a price, also if you keep needing to contact them for info the time will be logged and billed. It's very handy to have the information in front of you all the time. I was lucky my accountant had someone who dealt with Sage issues who I could email to confirm I was doing the right thing.
I'm guessing that you have done your accounting manually up until this point? So the crossover should be easy-ish.
Firstly use the practice data option to test what your trying to do it will not affect your accounts. Once you have tried in practice and your confident repeat in your company data.
Another tip would be BACK UP, BACK UP, You can never back up enough. At first do it before entering any new information its a helluva lot easier restoring to a previous point than correcting errors IME. As you get more familiar with the software you will have to do this less.
Journals are simply a way of adjusting payments that are not straight forward. I.E wages which are made up of NI, TAX, gross and net payment. One payment would come out of the bank but would need put into different categories for book keeping. Wrong entries where you have categorised something wrong can be corrected by your accountant with the use of a journal. Most things can be dealt with without using a Journal.
As for bringing stuff from previous financial years into the equation it would be a lot easier if you just deal with your current financial year, deal with the other stuff separately and bring them into the accounts later rather than having to change the tax rates depending on the transaction date. The vat will allow you to make changes before submitting so it will auto do the return for the current period and you manually make the adjustments for stuff you want to bring forward.
While getting your head round the software I found it a lot easier to deal with current rather than trawling through old, in fact by the time I got through the old even the new stuff was old if that makes sense
Another good tip is accounting is not a standardised thing when it comes to Sage, I have seen different people do the same transaction in different ways there's no hard and fast rules. The most important thing is that you and your accountant know why you put it there and hopefully your accountant will tell you which part you are doing wrong, journal to correct it at year end and advise you on how to do it in future.
Always remember that you are not the accountant your job is just to keep your records as accurately as you can and let them fine tune it before submitting.