I haven't tried this yet, but i want to experience. My question is tantra involve in some way sex or is just erotic massage.

I have seen Tantra ads in Eros, some don't offer sex and others maybe do, because too much eroticism can cause me want to have sex later.  

Those who have this kind of service can give some idea how it is? i also searched the subject but still have some doubts

Personally I think its advertised by women who have truly no REAL clue, training or idea what to do...  

I think sadly they use it as a ploy..

 
I do know there are truly a handful of sincere tantra goddesses. but you have to to find them.  

here is what I know  
achieving the big “O” is not Tantra’s main objective. Instead, you attempt to prolong the act, increasing potent sexual energy and intimacy with your partner. If you focus solely on the grand finale...
You have to also pay attention to the "space". Soft fabrics, think cozy yet sensual?
with little clutter, music, lighting, candles, and scents.  

Tantra is a lot about breathing, and trying to feel the energies. A lot like meditation...
Harmonizing your breath is one of the easiest ways to sync with your partner.  so most will have you and her Straddle each others lap and inhale while they exhale and vice versa. As your partner breathes out, you’ll find yourself taking their breath into and down through your entire body. As you exhale, consciously attempt to energize the breath. In this way, you’re sharing all of yourself with your partner.

ALOT of tantra they want you both eyes open and taking each other in while you have sex. It is not as easy as you might think! IT is very intimate when you do this.

and yes it is a slow and take your time type of experience lol  
A leisurely, slow build helps men control longevity and piques women’s arousal. The longer you linger in this process of building energy, the longer your session will last and the more energy you will build. Use this time to fully focus on each other. As in meditation, when your thoughts wander, gently guide your attention back to your partner and the magic of the moment at hand.

I think each experience is different BUT I do think it is TRULY hard to reach the levels possible in tantra with a guy you have seen once, or have no connection with... this is just my opinion though

I learned that the hard way.

I've met several tantrikas from this site and always found them to be very good.

Tantra fundamentally is pleasuring each other in erogenous area of your body. It is not finger up your ass or other holes. It is always done in a very relaxed environment and requires you to forgo all your biases, likes and dislikes and let yourself go and require utmost trust in each other.

Being that said, most of the ad for Tantra is crap really. If you want to really experience tantra, there are multiple day retreats and keep in mind, it is not fucking or group sex. Most people participate in the retreats with a someone they have known and trust and have an interest in experiencing Tantra. Google and you will find some good retreats.

END OF MESSAGE

lol Now i'm scared, because if that

How much time it is necessary for a good tantra? Also I read that tantra involve sex, will be very erotic sex and awkward  positions

from staples,     and a nice rubber glove over to hold it in place and some "elbow grease "brand anal lube is for. One my clients taight me that one. You know those like yellow rubber finger covers they use for counting money? if you have acrylic/tips you can still give prostate massages with these 2 supplies handy. I learn more from eager horny clients than i have from any where else, durin our sessions, most of them know more than we all do and you learn something at every session, i learned how to do pegging and the male prostate stimulation unexpectatly at dates with guys whom told me thats what they wanted to do durin a session with me, i am always game for new things but I am not always sure what they expect or need me to do . i have to say its nice to have a diverse menu and keeps things exciting.