Hello!!! :) I would like to know what Kamen Rider is about and if it's interesting??  I would like to know anybody's opinion about it (without any major spoilers).. 

Thanks!! :)

Did someone say... Kamen Rider?

Okay first I need to point out I'm not an authority by any means, but an enthusiastic (and relatively recent) fan. Kamen Rider, as you probably know, is a tokusatsu ("special effects") franchise that's been running in Japan since the 1970s. Not the same show for 40+ years, but the concept of a hero called "Kamen Rider (insert name here)" who has adventures, makes friends, imparts life lessons, etc.

Here I'm going to assume you're asking more about the newer, Heisei-era shows (starting with Kuuga in 2000 and up to the current, though almost concluded, Ghost). Basically they are year-long, weekly shows with themes that vary from show to show.  Sometimes the foe is an ancient civilization that wants to hunt humans; a nefarious organization that wants to rule the world; bleedover from a world in another dimension; vampire-like creatures that feed on people; or... well, it always ends up being a threat to all humanity by the end.  And the Rider fights that evil. Series generally start out being rather episodic-- establishing characters & relationships, what the Rider's special powers are, his signature moves and personality quirks, stuff like that-- by pitting the hero against "monster-of-the-week"-type foes.  As the show progresses though, a larger overall plot emerges until by the end (generally 48-50 eps) the Rider is fighting to save humankind.  And then it starts all over again the next week with a new series and a new Rider.

Of course he's (and yes so far it's all been dudes in the main Rider roles) not alone. The Rider will invariably be surrounded by friends/allies we've met along the way, and almost always be accompanied by at least one additional Rider. So even if you don't totally love the Main Rider, you may find your Favorite Character(s) amongst his companions.

The special effects? Hmmm, not amazing, not if you're going to compare it to Hollywood movies especially. At the end of the day, it *is* after all a show for elementary school kids to teens, and they are working on a weekly TV schedule and budget.  But, how do I put it-- that's not why non-Japanese adult fans watch Kamen Rider. It's for the (generally likeable) characters, comedic situations, and genuinely touching moments that are hung on this frame of "silly Saturday morning kiddie action show." * And once you see how utterly charming and downright enjoyable these shows can be, it's like the special effects become just fine for what they need to accomplish, it's not even an issue anymore.

*[Okay, some people also watch for the attractive actors, not gonna lie (I'm guilty as charged in that too).  And if you're a J-drama fan, it's fun to go back and see a lot of current "mainstream" drama actors getting their starts in tokusatsu shows-- not just Kamen Rider but also the Super Sentai series (where Saban gets the fighting footage they use to make Power Rangers) and the like.]

I'll be honest, I was a "regular" drama fan many years before I would even touch a toku series. I was always like, "Oh, those cheesy kids' shows they cut up to make Power Rangers? Pass." Through a total fluke, a few years ago I gave Kamen Rider W a chance and fell head over heels-- not just for that particular show (but boy, did I ever) but for Kamen Rider and the toku genre as a whole. It's only been a little over 2 years now and I'm almost to the end of my 9th Rider series. And I'm so absolutely happy I got over my drama snobbery and gave these deceptively simple-looking "kids" shows a shot because honestly my life is better for it.

/end Kamen Rider testimonial

Thank you for taking your time to write this big answer cause it helped a lot!!! :D
Kamen Rider is great. there are a lot of good series you could start out on. I would recommend starting with W or maybe Den-0 both are great series.   
I'm not that knowledgeable about Kamen Rider, and it seems that @kittyflumpin has covered up pretty much everything, but let me add that not all Kamen Rider series are geared towards children and young adults/teenagers, so you might want to check out what kind of thing you want to watch before deciding on an individual series. (E.g. Kamen Rider Ryuki and Kamen Rider Den-O are suitable for all ages; my personal childhood favorite Kamen Rider Black must've at least rated PG-13, and there's Kamen Rider 555 that seems to be geared towards more mature audience.)
Kamen Rider brings back memories :)

I personally recommend Kamen Rider Den-O (2007): Sato Takeru and...

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Kamen Rider Kiva (2008) Koji Seto


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Thank you guys for your opinions, it really helped me!! :D
I would posit that all the actual weekly-broadcast Kamen Rider series really are *primarily* meant for the younger crowd, with any extraneous adult audience being a happy (though not entirely unanticipated) bonus.  However (and most notably), what Japan finds (or found) appropriate viewing for children is sometimes pretty different from Western ideas. I get the impression they're more likely to respect the ability of younger viewers to follow a serialized plot (something that until relatively recently was lacking in most Western kids shows, which usually had to be strictly episodic so they could be aired/rerun in any order without affecting the story) and handle more complex themes in their media. So to Western eyes they seem like they're meant for a teens-and-up crowd because here they *would* be.

I do agree that some elements meant to appeal to the adult (& secondary) audience are frequently included in these shows-- scenes of (strictly non-sexual) semi-nudity of the attractive male cast (the first time I saw Kirihiko's bare butt in Kamen Rider W I almost fell off my couch tbh), the occasional BL subtext, etc., stuff that would mostly pass by the younger, primary audience unnoticed. After all, if mom (most likely to be the parent at home when the show is on) is also interested/invested in the show then perhaps there will be less resistance when faced with the merchandise in the toy aisle.  

Notably there *is* a definitely "adult" (probably more like Rated T for Teens and older) Kamen Rider series that aired just this year: Kamen Rider Amazons, a web series (initially streamed on Amazon Prime in Japan, no seriously) which has gorier situations (dismemberment, CG blood splashes, and body parts a-flyin' during battles) and cannibalism (or the functional equivalent) as a main theme.

To throw in my own 2 cents on what Rider to start with, W (read "Double") is a very popular choice and is generally considered to be one of the better Heisei era series (and it is certainly my personal first & favorite). *And* you get to see up-and-coming star actor Suda Masaki's debut role as the charmingly quirky Philip (points to my avatar).
For someone who is still deciding to start watching a Kamen Rider Series, I would suggest Kamen Rider Den-O. Aside from Takeru Sato as the lead here, the story is fun and interesting. And admittedly, I was really hesitant to watch it before as that the series has I think more than 60 episodes. But well, I think I finished it in a week, and I never even bothered or got distracted with any activities during that time. And once I finished the series, I felt like I want more of it. 
Oh! Thank you for the recommendation! :)