[Radek]

1. If you see an icon on your character portrait indicating that you can level up (a yellow cross in the upper part of the portrait) then open your personal screen (the one with the large portrait of the character). One of the, now active, buttons there is "Level Up". Press the button.
2. When you level up, you get HP, your saving throws and THAC0 improve and you get new proficiencies to improve your weapon skills. Well, it depends on the class how often these characteristics improve. For example, a warrior improves his THAC0 everytime he levels up, gets 1 proficiency slot per 3 levels and so on.
3. Your stats (STR, INT, etc.) will not improve at all. You can improve them only by means of magical items. That's why spending some time with generation of your character is worth it.
4. Changing class: if the (non boosted by magical items) stats of a particular character are enough good, you will see that a button "Dual Class" on its personal screen is active. Press the button if you want a dual class character. If the button is inactive then the stats aren't enough good and the character cannot change class at all. You can dual a particular character only once during the game. The chatacter must be single class character and a human. Multiclass characters or non-human races cannot change class.