First of all, I'm really new to RPGs, and I cannot find in the core rules or online how many ability score to give a character at level 4,8,12,16,20. The manual says "and possibly an ability score increase". Do you seriously get just 1? How do you reach some of the requirements for feats then, like 19 DEX? Also, the table for modifiers goes to like 50 why does it go so high if you cant get that high?
From the D20 Pathfinder SRD:
At 4th level, a character can increase one ability score by +1. This is a typeless, nonmagical bonus that cannot be changed once selected.
For example, a fighter with Dex 13 could use this bonus to increase his Dex to 14.
A character can also increase one ability score at 8th, 12th, 16th, and 20th level; it does not have to be the same ability score as the one chosen at an earlier level, and stacks with all other bonuses.
For example, the aforementioned fighter could use the 8th-level bonus to increase his Strength from 15 to 16, then use his 12th-level bonus to increase his Dex from 14 to 15, and so on.
To answer your specific questions:
Do you seriously get just 1?
How do you reach some of the requirements for feats then, like 19 DEX?
You'll usually have one or two stats fairly high to begin with. Choose a race that further increases this stat. Put multiple ability points into the stat over time.
Specialization is key: Choose the stats to focus on, and accept that the others will be lower.
Also, the table for modifiers goes to like 50 why does it go so high if you cant get that high?
The same system applies to both (humanoid) player characters, and NPC monsters. While your human fighter will probably never have 50 strength, a dragon might.