The table at the top displays all the skills along with useful bits of information about them - like Key Stat, your current Modifier, Rank and Cost in skill points to purchase a rank in this skill. A Cost of 0 means that you cannot purchase this skill unless in the Options tab you assigned a non-zero cost to these exclusive skills (usually the value is 4 or higher). Once you've clicked on a skill you can click on the Raise button to put a skill point into it (thus adding 1 whole rank to a skill that costs you 1 skill point per rank, or adding .5 ranks to a skill that costs you 2 skill points per rank, etc.), or the Lower button to remove a skill point from that skill (and reducing it a commensurate number of ranks - usually 1 or .5). You can click on the Max Rank button raise a skill to the highest rank you can - capped by the number of Skill Points Left for Class: value and the Max Class Skill Rank or Max Cross-Class Skill Rank values. You can filter which skills you want to see displayed - the Filter is the selection box right below the table and has selections of All, Class, Cross-Class, Exclusive and Untrained. The selection box at the bottom of the window is where you select for which of your classes you're purchasing skills - all of the skill costs in the table are driven by this selection.