so i cast teh spell on the thing i'll kill then kill it and it goes into the gem?
Yes, but you'll need a soul gem of good enough quality to hold the soul. Petty soul gems for example can only hold petty souls, higher quality soul gems can hold stronger ones, and stronger souls increases the amount of charge the soul can provide. Which when recharging your weapons means that you replenish more of the weapons charge. Aside from soul gems and varla stones, there are alaso some NPCs who offer an enchantment recharge service, but of course, this costs money, and you can only do it in cities, so it's not very helpful if you need to recharge you're items during a fight.
In any case, if you're going to use enchanted weapons, I'd suggest checking the charge of the weapon regularly and recharging it when it needs it, so you're less likely to suddenly run out in the middle of a fight.