I wouldn't say you need to reroll. Dark Souls is a game mostly about skill and tactics - more so then proper builds and items, in my opinion. The one legged demon you mention CAN be defeated even if you're in shit gear, and even if you gain more levels or get better gear you will die very quickly if you get hit. Make sure to avoid it's attacks and if it is going to hit you, you better have enough stamina to block it. Seeing as it's not necessary to progress in the game I would say you did good in skipping it. You can still farm up souls and try at a higher soul level, as it will help you some. Make sure to get a lot ot VIT and END, it really is useful.
What you should do is to upgrade your weapon more. A spear is not a bad idea at all, they are quite easy to use and forgiving, seeing as you can attack from behind your shield (which is not ideal, but can be safe).
The Hydra you're talking about is basically about knowing the tactic and to be honest, I never figured it out on my own - I youtubed it and then proceeded to kill it on my first try. I am of the opinion that Dark Souls have a lot of those bosses though - extremely easy once you find out one thing you need to do, and if you don't, it's impossible. This is at least true for the Hydra - for me. Again, you do not need to kill it, and I would wait with it.
What I would do if I was you was to train on blocking and avoiding attacks on regular dudes, as well as learning to backstab and riposte. These are very useful skills, not necessarily for bosses perhaps (except the avoiding part), that make the game much more manageable. That way you can also get some souls so you can upgrade your gear, as you should have at least some titanite. Also note that regular or starting weapons can become very powerful if you just upgrade them. Your shield is a bit shit though. Try looking around the area sort of away from the Golems and the Hydra. Don't go up as there is a badass dude there, but do look around the general paths there. There is a very useful shield on a corpse around there that is certainly much better then what you have.
I suggest you skip the garden and just go straight to the Depths to continue progressing in the story. Look on the key you got from the Gargoyles for a hint.