I may be able to offer some assistance in this matter as I have studied and taken a class in martial arts. Aikido is known as an internal martial art, focusing on the spiritual aspect of one's strength rather than pure and brute force. It is a series of throws to counteract and use one's own strength against them. While intended not to hurt the attacker, the moves can be dangerous. In irony it is the martial art with the highest rate of injuries, being as, if not executed properly the throws can lead to quite serious personal injury such as sprains and broken legs.
I myself have taken lessons in karate and found it to be enjoyable. While it is based in the Japanese Shinto religion, most American dojos teach watered down techniques. The seemingly "sugarcoated" versions are not as intense or rigorous as their Japanese counterparts, while still a good form of physical activity. From what I understand they have many competitions one may attend regardless of one's belt color. There will, as with mostly any sport, inariably be people you do not like. Some talk about other religions and such however depending on where you live, there may be Christians as well.
JuiJutsu is a more "hard-core", so to speak, type of Judo. Brazilian JuiJutsu is a martial art I am not familiar with, however I do know that it consists of a series of throws and counterattacks. Very likely the most Sumo-like martial art. It is a bit more dangerous and physically challenging thatn karate.
Taekwondo is a Korean martial art which most hard core martial artists find to be too sugarcoated. They follow a belt system. All techniques at children's dojos are mostly geared toward defensive purposes. However, in the adult classes one can find a wider range of techniques. My own opinion of Taekwondo in America is that it is somewhat mainstream and loses its traditional values. Most likely no need to fear religious conflicts.
As far as religious conflicts I can't tell you what is right and wrong. Your conscience should be your guide as far as this aspect. If you get into the more serious and less Westernized martial arts such as Ninjutsu and Kendo you will see the shinto issue rear its head. I personally steer clear of those, also martial arts involving meditation. Best not to let one's mind go where it doesn't belong. Especially mine, I have a hard enough time keeping track of my shoes!
Some martial arts practice the use of chi but that's mostly in Tai Chi or Kung Fu so I don't hink you have to worry. Also do not get yourself into a commitment. Call around the local dojos and ask if you may come in for a trial session. Most dojos will be happy to oblige if they do not offer it. Remember that most martial arts dojos will want your parents to sign a contract so if you don't feel comfortable or aren't sure about which one you would like to choose, don't feel pressured to join. Also, when I first became interested in martial arts I had a glassy-eyed view that after a few kendo lessons I'd be Kenshin.
Martial arts are arguably a sport. I had a Taekwondo instructor tell my father that you're not going to be able to flip a 300lb guy but you will learn practical techniques that may serve you well. MY best advice would be to do your research and be realistic. If you do those things martial arts can be great fun and a very rewarding extracurricular activity! God bless