I'm not sure exactly what is hanging you up but will give it a shot.
Tickets are pre-sold by time slots so they can try and regulate the amount of people coming in at any one time. So say you're trying to buy tickets to the Uffizi for Sept 14th?
Choose Sept 13th from the b-ticket Uffizi calendar and a new page comes up.
That page will show available tickets for dates beyond Sept 13th. Click on the the green "tickets" button for Fr 14.09.2018.
What comes up is a list of open time slots to the museum for that day +the option of purchasing a cumulative ticket which includes entry to the Pitti Palace and Boboli Gardens in addition to the Uffizi (see my note below). It also includes pricing tiers for adult, full-priced tickets, reduced price for EU students between certain ages, and reservation price for children under 18. There are also annual tickets, which can be ignored. So, you just chose the number of each type of ticket you want under the time slot you'd prefer, click the green "in the shopping cart" button at the bottom of the page and proceed to checkout.
Do the same thing with the b-ticket website to buy tickets for the Accademia: go to the page for that museum (https://webshop.b-ticket.com/webshop/webticket/bestseatselectbyblock?eventId=20175&el=true). choose an available time slot and number of each price tier: "Intero/Full" for an adult ticket and "Gratuito/Free" for each child under 18, You'd only buy Ridotto/Reduced tickets if you are or will be visiting with an EU student between ages 18 to 25.
Click on "Terms & Conditions of the Service (please read carefully)" at the bottom of ticket-selection pages for helpful details, such as picking up your tickets (opt, on the ordering page, to pick them up at the museum versus having them shipped).
The Pitti Palace and Boboli Gardens are in a completely different location than the Uffizi. Look them up on a map and do a bit or reading about them if you're interesting in purchasing the Uffizi ticket which includes these.