Best thing is to find a friend to train with. If you can't manage that then best thing is to do something you like else you will just give up.
If you are interested in martial arts then all of them can be taxing, just depends on how much you effort you want to put in and what you are looking to get out of it skill wise.
Assume that you can walk up a couple of flights of stairs without getting out of breath.
From personal experience, karate is the easiest and initially involves the least contact but to be honest on the street you will slaughtered. Tae kwon do, kick boxing, kung fu, jkd, mauy thai, etc as all striking based. a good dojo will get you sparing fairly quickly. Expect a lot of repetitions of basic moves. Out of those mauy thai and kick boxing have the most self defense value.
Ju Jitsu is based on blocking, locks, chokes and throws. Quite technical, you will be in physical contact from the outset and believe working with a 18 stone guy will quickly wear you down even though its based on using your opponuts energy against him.
Brazillian jujitsu is the 'in' martial art now with its popularity in cage fighting, I still have my doubts about its use on the 'street' but its mostly ground based and using your weight and positioning to control your opponut. Its also VERY cardio based, even 15 minutes will live you knackered. I would also be careful if you have a bad back with JJ.
I would say all good place welcome beginners and will offer at least one or two free lessons. Look for a place without ego, class size of at least 10 if possible (more varity keeps it interesting) but instructor to pupil ratio of 8 or less if possible. Everywhere I have trained, they have cross trained in more then one art. They should be open to any questions you have.
I would definately say try and find somewhere where you are going to have fun.