In DSII, you no longer ascend a weapon to a Boss weapon using a boss soul. Instead you 'trade' the boss soul and some additional souls to 2 specific NPC's to get Boss weapons.
The first is Straid of Olaphis who can be found petrified in a jail cell guarding a bonfire at the top of Lost Bastille. Using a Fragrant Branch of Yore will allow the player to trade boss souls with him. To be able to trade with him, you need at least 3 Intelligence and 3 Faith. This is a one-time requirement. If you later redistribute your stats with a Soul Vessel, you can still trade with him so long as you have talked to him with the required stats at least once.
The second is Weaponsmith Ornifex but you must unlock her before you can trade with her. To unlock her visit Shaded Ruins, to the right of the bonfire is a petrified enemy, you can jump from the ramp onto the raised walkway and use another Fragrant Branch of Yore on him and he will drop the Fang Key. After getting the Fang Key, drop down in the pit next to the giant Green Basilisk and use the key to open the door. This is the first time you meet Ornifex and she will tell you she is moving to her home.
She will then move to Brightstone Cove Tseldora, in the room next to the Brightstone Cove Bonfire. Talking to her here finally allows the player to trade the rest of the boss souls to her.
Keep note that depending on how many bosses you have fought before unlocking Ornifex, will be the number of boss weapons she has. She will not show you boss weapons for bosses you have not beaten yet so always come back to her as she will craft the weapons for all bosses in the second half of the game. Also, once you free her, Ornifex will perform one, and only one, Boss Soul trade for free.
You won't be able to level up at all until you get past the Things Betwixt area -- essentially, the tutorial. Past that and through a narrow cave is a town called Majula.
When you first enter Majula, there is an area to the right which has a tall tower overlooking the sea. To the right of that, slightly down the cliff and by a tree, there is a green-cloaked woman known as the Emerald Herald. Once you speak to her, she will relocate to the Majula bonfire. Thereafter, you can speak to her and level up at that bonfire. Unlike Dark Souls, this is the only place you can spend souls to level up.
When speaking to her at the Majula bonfire, be sure to wait for a menu to popup. You may have to speak to her several times before the option becomes available.
As a side piece of advice, if you want to save your souls until you can first spend them, don't go down into the area with the two trolls. This is a difficult encounter for a new character. You can bypass that and come back to it later to fight them.
Best Answer
It's a little late in the game, but the Black Gulch is a good area to farm titanite chunks and large titanite pieces. The best part is, there is close to zero risk of you dying from this, once you get the hang of it.
Shoot the black tar pits with a fire arrow, and the creatures will die from fire. When they are dead, run up, collect items, and repeat. Use a bonfire Ascetic at the first bonfire, to get the mobs to respawn. You can shoot at the tar pits a little after the first bonfire. If you use a bonfire Ascetic at the second bonfire, the boss will respawn, but not the mobs.
To farm the second area, stand at the exit of the second bonfire, and you can ignite the 5-6 tar pits quickly and easily. Best part is, when you replay it in NG + later games, this will not make the area significantly harder, because you can use the same cheesy tactic, to take out the mobs, without danger to yourself.
This will also net you quite a lot of souls. Remember to equip the Jester's cap: http://darksouls2.wikidot.com/jester-s-cap + Covetous Gold Serpent Ring: http://darksouls2.wikidot.com/covetous-gold-serpent-ring