To set up an offer to buy anything, for action during that process, before the end of turn you click the third button in the main menu (with the coins icon, called "Give Trade Orders") near the bottom of the screen then the left-hand tab in the "Orders" column. You may offer to buy anything listed, up to a maximum of three items in any turn initially but six after you have researched Trade Fairs.
After clicking "End Turn", when the program has done your selling, your resulting cash balance will show at top left (giving you a rough guide to how much you should restrain your buying if you will be in debt).
You will receive other nations' selling offers (if any). You get presented with one offer after another and may accept the whole offer (if enough merchant marine space is available) or a lesser number or may reject the deal. Offers follow the order of items on the "Give Trade Orders" list. Whether accepting or rejecting, you may save yourself time by checking the box at the left to receive no more offers of that commodity this turn.
At any time in the process you may click on any of the highlighted commodities on the right-hand scroll to see who is offering and who is bidding, what order each offeror will choose to talk to bidders, and how many merchant marine holds each bidder still has free. That usually gives you some idea of what you will be offered later, so that you can plan to reduce current acceptances if you want enough holds free for a more desirable commodity likely to be offered later.
Finally you see the "Deals" page for details of what you bought and sold and some of the other factors affecting your cash flow. Turn the page at the bottom in the usual way if there's an indication of a subsequent page. The right-hand scroll still contains information about bidders and offerors but nothing about remaining holds.
You may re-inspect the Deal and Market Summary later in the turn from the "Help" menu reached from the Information button at top right of the map screen.