Takes time & patience.  If you're gonna bring a new kitten into the
house.... Here's a few pointers:
- Set the new furball up in one room, where you can close the door.  food,
bed, litterbox, water, toys, et al....
- Introduce them to eachother in short, supervised sessions in afore
mentioned room.
- Pull the occasional 'swich', put your current cat in the room & allow the
new one to explore the rest of the house.
- During the supervised visits, play with them both.  Best to get a
comletely 'new' cat toy to do this with so it does'nt yet smell like either
of them.
- A tactic that worked for me....  Bathe them both, one after the other &
then cuddle up with both of them, on the couch, under a blanket, while they
dry.  This was the breaking point for my kittys.  By the time they were
drying out nicely, they were grooming eachother & ended up napping together
curled up in one damp little ball.
- Getting two cats to accept eachother can take weeks (or someimes months,
depending on personality).  They will probably fight & wrestle... Let this
happen, as long as no one draws blood & no fur flys. It's part of their
normal 'play' routine & it establishes pecking order.

Good luck!  Having two cats is GREAT! Once you get through the adjustment
period, they become a team. They play with eachother, groom eachother & keep
eachother company when you're not around.
Mathew
Butler to 2 kittens: Chablis & Muscat
En Vino Veritas