it's a tough game at first, but your efforts will pay off as you progress. you'll definitely enjoy it.

1. thief may have been a tough choice, yes, however, you can obviously still continue as classes do not mean much in this game. they're just a starting point (slightly different points spread, and different starting equipment). you can make any character you wish as you upgrade your soul level. my advice is to reroll as a royal/knight. they have better starting equipment which will be a bit more forgiving for someone starting out.

2. again as mentioned above, classes do not matter. you can choose to play any combat style regardless of class as long as you put your souls in the correct stats.

3. no, you don't have to play in that order. it's an 'open-world' type concept, so pick and choose where you go. there is a general progression you can follow however - 1-1, 1-2, 2-1, 3-1, 2-2, 4-1, 5-1, 2-3.. etc. but it really doesn't matter, you'll figure it out by playing. this game has a larger focus on your skill rather than your character's in game skill (soul level/equipment/etc), so you can pretty much go anywhere and potentially kill anything.

4. don't worry too much about world tendencies for now. in short, depending on what you do in the game, different events occur. but you'll have time to delve into this later on.

5. this is a perfect example of word tendency, although i doubt you triggered it. if you die in human form (or kill certain npc's), your world tendency goes towards black, and therefore, enemies drop less healing items. in your case though, i think it's an anomaly. don't worry, you'll find plenty more grass as you progress. and if you seem to be running short, just take it slowly.

6. soul level is your characters 'level'. you upgrade your soul level by speaking with the maiden in black in the nexus. you trade collected souls in return for upgrading your stats. as you upgrade your soul level, you character will become 'stronger', will be able to wield better armour, weapons, etc. this will allow you to play towards a certain build/playing/combat style you mentioned above. and note that your weapons/spells will 'scale' with your stats. so a dagger, for example, will do more damage for someone that has high dexterity. so all of this ties together with soul level.

other advice: make sure you explore the worlds thoroughly, especially 1-1. there's an item there that will definitely make your life easier.

other than that, good luck. the game is very rewarding if you stick with it. seemingly impossible tasks will end up being trivial if you just look at them in a different way. :)

and if you're stuck, just keep asking questions.

edit-typo.