The table in the upper-left contains the list of "Available" classes. Next to the word "Available" is a selection box that lets you pick which "view" you want. It is this view which will be used to determine the structure of the table below it. The same is true for the "Selected" classes which list the classes and levels of your character. By clicking on the Filter icons in the toolbar (the icons with the magnifying glasses) you can filter which classes are included in the table. You can add a level of a class by double-clicking on it, right-clicking on it or clicking on it and the on the right-arrow button. Similarly you can remove levels of a class by double-clicking on it in the "Selected" table, or by right-clicking on it there, or by selecting it and then clicking on the left-arrow button above the table. As elsewhere in PCGen anything you don't qualify for is in red italics. The Pre-Reqs columns lists the pre-requisites of the classes. Before you add any levels you must first select an alignment in the lower-right corner of the screen since some classes gain abilities which are dependent upon their alignment.
The info-pane in the bottom-left will give you details about any class you click on - including its HD, BAB progression, save progressions and more.
The bottom-right section gives you class-related information about your character. You can click on the HP button to see your level-by-level hit-points (you can adjust them there as well). If the Max HP % field is anything other than 0 that % of Max HP will be assigned at level advancement rather than randomly generation. Your current number of optional feats to spend, your remaining skill points, base reflex, fortitude and will checks are also displayed. Most base classes cannot advance beyond level 20, and most prestige classes cannot advance beyond level 10. To ignore these caps you can click on the Ignore Max Level checkbox. The layout of this section may vary slightly from game-mode to game-mode. The experience field is editable and indicates how many experience points your character has. If you'd rather have PCGen do the math after and adventure, you can simply click on the Adj XP button and type in a value which PCGen will then add to your experience total.
There are "sliding dividers" between the class tables - you can click on this divider and slide it back and forth to your preferred location. There's also a divider between the upper-tables and the info sections below which you can drag up and down to change the viewing area of each. There's another divider between the info sections. This combination of 3 sliding dividers is common to most of the character-related tabs to allow you to change how much screen real-estate is devoted to each section. On most tabs there's a button which has 2 circling arrows that will reverse the orientation of the views between horizontal and vertical. This gives you many options on the layout of the tab.