I'd say give 15# a whirl to start. Do you have a video? the recommendations have been posted many times if you want to do a search. but my fav right now is Lauren Brooks w/o.
I started with 10# and still use it (and get a really good workout out of it). I bought a 15# because I thought I was ready to progress, but ended up hurting my back and shoulder when I went that high, so I went back down to 10#. I ordered Cathe's weighted gloves, so I'll probably use those to add a little bit of weight without going too high.
If you are just starting out and using 10# or 15# you should be ok using just 1 KB for a workout. Kettlebells are just like weights. You can do some movements with a heavier kb (swings) and some movement with a lighter kb (TGU or windmills). I have kbs that range in weight from 10# to 44#. When I do a kb workout I'm switching back and forth between lighter and heavier kbs depending on the movement.