I'm new to Pokemon games series, I haven't played any game of the series
and these games sound nice, Can anyone here help me with which one of them should i begin with! (I only have a 3ds)
Well, you'll be able to play 4th and 5th Generation games, and also the upcoming Pokemon X & Y.
But there's some great ones worth playing from those generations.
Platinum has some good post-game content and it is much superior to Diamond & Pearl, remember that if you want a good Sinnoh game definitely go with Platinum. Diamond & Pearl aren't really that great because the speed was so slow and the Sinnoh Dex in those games was much inferior to Platinum's. Remember, don't get Diamond/Pearl, get Platinum.
HeartGold & SoulSilver give you that nice sense of nostalgia from the original Gold/Silver/Crystal, and it also has some neat post-game content, and also new improvements since the original G/S/C. It also has the ability to allow your Pokemon to walk around with you, and it even has the Pokeathlon.
And there's also both Black & White as well as Black 2 & White 2, which have really great graphics. Black/White may not have as much post-game stuff as other games but the story is among the best in the series. Playing the original Black & White before playing Black 2 & White 2 can help give you more knowledge on the Unova story.
Black 2/White 2 however, have loads of stuff you can do and there's loads of Pokemon you can catch. The Pokemon World Tournament, Join Avenue and Black City/White Forest are all very addicting imo. And since you have a 3DS, you can also download the Pokemon Dream Radar app which you can then catch loads of good Pokemon with, such as the legendary Pokemon Tornadus, Thundurus and Landorus.
In my personal opinion, all are great games but maybe someone else might want to ask as well so you can eventually decide which of the games you want to play.