As you level up, your character gets better. Not just their skills, either. With enough practice, their base attributes can improve, their saves can go up, and they can get new feats. These upgrades are the same for everyone and are linear in design.
Base Saves and Attack
Base Saves
Your saves are Reflex (Add DEX modifier), Fortitude (Add CON modifier), and Will (Add WIS modifier). As you level up, you will find these increasing rather rapidly. You can freely assign the base saves to whichever ones make sense for your character. Note that this list is the total you have at the level, not cumulative.
Base Attack
Your Base Attack is how accurate you are. This determines if a spell, arrow, or sword will hit. It replaces Attack, Ranged, and the combat aspect of the Magic skill. If you have a / in your base attack, that means you get to make more than one attack if you take the Full Attack full-round-action. These attacks take the bonuses before and after the slashes. For example, a 7th level offensive character can make 2 attacks in a Full Attack, one attack at +6 and the second attack at +1.
Defensive Characters
Level 1: +0, +2, +2 Base Attack: +0
Level 2: +0, +3, +3 Base Attack: +1
Level 3: +1, +3, +3 Base Attack: +1
Level 4: +1, +4, +4 Base Attack: +2
Level 5: +1, +4, +4 Base Attack: +2
Level 6: +2, +5, +5 Base Attack: +3
Level 7: +2, +5, +5 Base Attack: +3
Level 8: +2, +6, +6 Base Attack: +4
Level 9: +3, +6, +6 Base Attack: +4
Level 10: +3, +7, +7 Base Attack: +5
Level 11: +3, +7, +7 Base Attack: +5
Level 12: +4, +8, +8 Base Attack: +6/+1
Level 13: +4, +8, +8 Base Attack: +6/+1
Level 14: +4, +9, +9 Base Attack: +7/+2
Level 15: +5, +9, +9 Base Attack: +7/+2
Level 16: +5, +10, +10 Base Attack: +8/+3
Level 17: +5, +10, +10 Base Attack: +8/+3
Level 18: +6, +11, +11 Base Attack: +9/+4
Level 19: +6, +11, +11 Base Attack: +9/+4
Level 20: +6, +12, +12 Base Attack: +10/+5
Offensive Characters
Level 1: +0, +0, +2 Base Attack: +1
Level 2: +0, +0, +3 Base Attack: +2
Level 3: +1, +1, +3 Base Attack: +3
Level 4: +1, +1, +4 Base Attack: +3
Level 5: +1, +1, +4 Base Attack: +4
Level 6: +2, +2, +5 Base Attack: +5
Level 7: +2, +2, +5 Base Attack: +6/+1
Level 8: +2, +2, +6 Base Attack: +6/+1
Level 9: +3, +3, +6 Base Attack: +7/+2
Level 10: +3, +3, +7 Base Attack: +8/+3
Level 11: +3, +3, +7 Base Attack: +9/+4
Level 12: +4, +4, +8 Base Attack: +9/+4
Level 13: +4, +4, +8 Base Attack: +10/+5
Level 14: +4, +4, +9 Base Attack: +11/+6/+1
Level 15: +5, +5, +9 Base Attack: +12/+7/+1
Level 16: +5, +5, +10 Base Attack: +12/+7/+1
Level 17: +5, +5, +10 Base Attack: +13/+8/+3
Level 18: +6, +6, +11 Base Attack: +14/+9/+4
Level 19: +6, +6, +11 Base Attack: +15/+10/+5
Level 20: +6, +6, +12 Base Attack: +15/+10/+5
Base Save Example
A Level 6 Character with 12 (+1 Mod) DEX, 14 (+2 Mod) CON, and 15 (+2 Mod) WIS decides to put +5 in Reflex (DEX), +5 in Will (WIS), and +2 in Fortitude (CON). Their total saves are:
Fortitude: +4 = +2 +2
Reflex: +6 = +1 +5
Will: +7 = +2 +5
Simply put, you add your characters modifiers to the saves. DEX for Reflex, CON for Fortitude, and WIS for Will.
Ability Score Increase
Every 4 levels you get one Ability Point to place in a skill of your choice. You get Ability Points at:
Level 4
Level 8
Level 12
Level 16
Level 20
Feats
Feats are what set your character apart from everyone else. At first level, you get 2 Feats, then at level 3 you get your 3rd, then one feat every 3 levels after. You can choose bonus feats from an offensive or defensive list depending on your character class. You get these feats at levels 2,3,4 and every 2 levels after.