im starting to read the coral rules and im a lil bit of confuse, when and how to use... LOF and CLEaR LOF, please help me on this

The line of fire is simply the imaginary line from the center of one square (and thus its contents) to the center of another square and its contents.

Depending on what is in the squares intersected by the line, the line of fire may be in one of three states.
1) Clear
2) Hindered
3) Blocked

The line is considered to be the most restrictive of whatever it crosses.

To develop a little bit :

Clear : the LOF is clear when the line between the two squares (the one your character occupies and the one your target occupies) doesn't cross anything except clear terrain or water terrain.
Hindered : the LOF is hindered if the line crosses a square of Hindering Terrain, including the square your target occupies but not the one your character occupies. Hindered LOF allows your target to modify its defense valu by +1 against any range attack your character might want to make. A hindered LOF doesn't prevent the use of any effect that require a LOF. Some powers like Stealth turn an otherwise hindered LOF into a blocked one
Blocked : a LOF is blocked if teh line between the two squares crosses a blocking terrain, a wall, or a character. A blocked LOF prevent any raneg attack and the use of any effect that require a LOF.

This is true wenever your character and his target are on the same elevation level, as do all other features and characters.
If both your character and your target are elevated, no grounded standard character or grounded terrain feature will affect LOF. Giant size and Colossal size users will block LOF thu.
If both are grounded, then elevated terain will block LOF.
If only either your character or your target is elevated, then grounded hindering terain will not hindered LOF, except if the target is occupying it (even if it is grounded and your character is not). All elevated terrain will block LOF except the square occupied by the target (if your character is grounded).

Grounded Giant size users and Grounded Colossal size users draw LOF to elevated characters as if they were elevated too, and elevated characters draw LOF to them the same way.

I think I've covered it all. If somebody see a missing point or a mistake, feel free to correct me.

Glancing through Captain Krueger's list seems to get it right. I recommend checking out the line of fire table on the last page of the Core Rulebook (link in my sig). It illustrates just about any line of fire and how any terrain/game effect can impact it.

There are several Speical Powers (and many on the PACard, at least the Fantastic Four iteration) that refer to a "clear line of fire" and sometimes I wonder, too, how hindering terrain affects these... has this been cleared up? in these cases, "clear" does not mean "unhindered"...

That is covered in the rulebook:

I think that one reason why they started using the "within line of fire" terminology was to eliminate this confusion.