"irregular geometrical form terrain" means nothing, and "approach" is so vague you might as well be asking "How do I architect?"

This question is too general to answer. Can you give an example of what you're struggling with? Or rephrase your question to be more specific?

Ditto tduds. "Approach" can mean either a design strategy or a driveway/walkway. "Irregular geometrical form terrain" probably means a sloped or otherwise challenging site, but if your intent is something more specific, please rephrase.

Okay, i'll explain better, imagine you have a irregular pentagonal shape, Look at the picture below : the grey area is your terrain, the red lines are your roads.

let's say you're asked to design something, i don't know ... a commercial hub or a residential building. how you start drawing, (forget about the analysis & everything) just as a form, how you'd start. i struggle a lot, usually i create an axe that is parallele to one of the lines, & see what i can get to with it, if i'm not satisfied i try something else. but i'm sure that's not the only possible way, i've seen many projects that are like... designed without following any axe, but still gives that integration feel. i hope i made it at least a minimum clear. i'm sorry if my english is bad or my terms are not clear. i study in french & its hard for me to translate that.

As with every project start with a thorough site analysis.

views, sun exposure, surrounding buildings / features, traffic and circulation, natural features, setbacks and zoning requirements. 

From there you should be able to gain a direction of where programmed spaces should be located on the site.

At this point the form of the building should start to become more apparent.  The final form will of course depend on the designer and client.  I myself am a form follows function person.   

What's a normal form terrain, and where is it normal for?

Like are we talking large drumlins in Georgia, or deserts in Minnesota?

So after all we were talking about; i'm still on the same point; i know globally what you people trying to say, i appreciate it, thanks you. now what's annoying me is that feel of "lost", the problem isn't about the terrain analysis, or the theme analysis or anyhing, its more about "competences", i have problems with volume & shape compositions i guess, that's why i get mostly lost. i miss something that is probably a basic thing. if anyone know a book that could help please let me know. any other advices are appreciable, talking with you will only increase my knowledge.

Thanks you again.

Stop thinking. Start drawing. 

You're expecting to have a flash of inspiration, from which the solution will just flow naturally. Trust me when I say that's not how any of this works (as I've learned by being in your position many, many times before). The best way to overcome a creative block is to just start doodling bullshit. Get out a piece of trace and draw lines. Put another trace over that one and draw more lines. Think critically about what's pleasing, and why. More trace. More lines. Eventually something coherent will begin to emerge. Think about it more. Draw it again. And again. And again still.

*Then* you'll be doing architecture.

Here's a curveball for you, now is a good time in any major city to notice a lot of construction activity.  Take particular note to how in many projects they just bulldoze everything in the way, dig foundations, and do whatever they want.  Get some dimensions on that irregular shape of yours, find the setbacks required in your area, and  plop down a rectangle, long face roughly N/S and take a gander.  

Personally I'm having trouble understanding whether you're having issues "designing in a grassy field" or "fitting into a small site".  There were definitely studios addressing both issues, but they're really fundamentally different.  If it's the latter, again, the shape doesn't matter, you're going to try and use all of it.  Build to the lot lines if allowed.  Provide some street scape of some type and think about your front door a lot.  

Well, its more of designing in a dusty field than a grassy one lol. small sites let me fill all of it, which usually doesn't make me scared that much. getting out of space can be bypassed by going vertically. in larger ones, it's a bit tricky, you're like somewhere in a desert trying to find anchor point here & there. i usually never think of front door or entrance mark or whatever its called. usually when i start designing, i litterally can say : "oh here, it's the entrance. i can see it, it have to be here". i never thought of it as a part of the design process... probably a mistake i've been making... i'mma take a look at this, it may unblock my actual situation. Thanks you.

it seems like you're getting stuck on the site boundary as a geometric figure. which it isn't, it's an imaginary thing. look at the real views of your site and let that guide your design.

particularly in the kind of "open field" you seem to be imagining, the architecture has no meaningful relation to the site boundaries. it's purely about developing a form that suits the use of the building and a more "eye view" understanding of the site.

that looks like a kraken, not architecture wtf xD