PayPal API with vb.net the real basic HOW TO?

Hi, (sorry for my english), and sorry i am not really knowledgeable

i am making a website(asp.net-vb.net visual studio ultimate 2010),  i succeeded in making it fully functional (i hope), anyway, i made a only a simple paypal button where a user clicks--->goes to paypal---> pays---->re-directed back to my website(its about registration for medical seminars). and that is it.
Still i tried so hard to learn this PAyPal API thing, iread about it and i could not understand a bit! i am sure some folks will surely agree with me that in paypal (live or sandbox) it is a huge amount of information in a really huge website with huge information to go through(which i did by the way) but then again i find my self lost and not understanding anything
not knowing a place to begin  reading and learning to do actual work or see a (functional understandable sample) or practice, so,  i would (sorry) say that in my own humble opinion & maybe due to lack of experience as well i could not understand a thing from paypal help...anyway
here is what i know:
you get API cridentials from paypal-easy
code or API is only in several languages (nothing in vb.net which i based my website on)
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here are my questions or problems:

****what will getting API on my web do? (meaning staff will pay from within my website??? no need for jumping to paypal and back???)

****how can i code/add/integrate this API thing into my website and what do i need? (i searched and researched, only found people say get API--> put code or dothis and that but not HOW or provide corresponding samples that may or can assist in my situation)???

**** is it only API that i need beside working on my webpage design and all but from paypal do i need only APIs??

**** searched for tutorials as to how add the asp.net(VB.NET) code for the paypal-IPN page to no avail.

any help in this regard will be much much much appreciated i wanna learn this form the ground up, actually to give you guys  a glimpse of how less i know lol, i read about all or this and forgot what is an API actually-really is. (although i am using it in my website from TWILIO to send confirmation SMS to participants.)

Many many thanks in advance and apologies for the long writing...