1. It is worth it if any of the beliefs support your playstyle. As a general rule, if you can found a religion without sacrificing too many other things, it will always be worth it. Your beliefs can help your growth, culture, happiness, production, science, and tourism some.
2. All religions are the same (blank slates), but the beliefs you choose define what it is.
3.. If you have no religion then it is super-easy for you to be converted for the most part. One missionary use will do the trick. Even if you have another religion if the AI uses a great prophet it will erase all pressure and plant to new religion. Prophets are OP like that: an inquisitor and missionary rolled into one.
4. Pressure is mainly calculated by the number of nearby cities of that religion. The holy city exerts more than others. If the AI has more than you then they have already converted a lot of cities nearby. Two beliefs increase the distance pressure acts, and the amount. Also building the grand temple doubles pressure given by a holy city. If you want to defend yourself against high pressure you will have to either fight the war abroad with missionaries/prophets and convert nearby cities or keep popping inquisitors to erase the effects over time in your own cities. (inquisitors banish all religions but the one the were made from and erase all pressure built up over time there)
5. The AI puts loads of effort into it because some are programming to have a high religious "flavor" encouraging them to spread it and compete with other religions. As a general rule, your founder belief becomes more powerful the greater number of cities under your religion (though some only benefit CS or the act of converting). The AI will get more gold, culture, faith, etc. the more cities it converts--you will too if you do spread your own.
End of story. Religions tend to benefit whatever you choose so they are always helpful as long as you don't waste so much time on them you hurt yourself in other ways. If you're just starting, try to pick the founder belief "tithes" as it is the easiest to see the effect of. Get more gold per turn the more citizens are your religion worldwide. On some good games I've been raking in about 300 extra gpt from tithes later in the game. It's a nice pick and the AI ignores it for some reason so it is easy to grab. Experiment for yourself though...there is no consensus on how best to use a religion, they are flexible on purpose and a nice feature.
