How much can you spend? OS you want to use?
Are you comfortable drawing on a standart graphical tablet, or would you like a tablet / screen combo (screen with digitizer layer like cintiq)?
Depending on what you want need, your hardware could be many things...
1) cheapest is to get a Bamboo Wacom Tablet, or a similar, maybe even cheaper no-name (= non wacom 
) tablet, and hook it up to your existing Windows/MAc/Linux laptop.
2) if you want more oomph from your graphical tablet, the Intuos line will give you more precision and additional express keys as well as more selectable sizes.
3) You could try to pick up a mobile tablet with digitizer layer... just forget the Samsung android ones, besides lacking software they are NOT fast enough for serious art work.
The Windows Surface generally gets good reviews from artists, as you get a full fat win 8 machine with digitizer and decent hardware for aroun 700-1000$.
Get the surface 3 if you want to try the new digitizer, don't be fooled by the lower specs than on the older wacom digitizer, people said its just as good as the old wacom option, just with a heavier pen, and no jittering around the edges like what plagues all wacom digitizers. That said, you can still pcik up a surface 2 with wacom digitizer if you search around.
4) Premium options would be the cintiqs from wacom. Obviously the stationary ones are to big and unwieldy in your case, but the mobile ones might be interesting. As long as you are prepared to pay around double the price you pay for a surface 3, you will get a machine with just as much oomph, that has been better tuned for creating art. The more precise digitizer layer might make a difference, or not, depending on your skills and needs, but what I personally like are the cintiq style express keys and rocker rings. Missing programmable physical keys on all the other mobile tablets are a MAJOR pain in the ass for creating art, wacom made just the right decision by making their mobile tablet fatter and keeping the physical controls from the stationary cintiqs!
You can get a an android mobile / stationary cintiq tablet for windows hybrid, if you feel comfortable that you can live with the limited software support on Android, with the benefit of saving quite some bucks...
For the software, your cheapest option is Gimp... if you are looking for something more specifically tuned for drawing and sketching, have a look at sketchbook pro and manga studio / clip studio, both available for under 100$. The latter, while being created as a program for comic drawing, is especially cool for mobile use because of its stroke stabilizing... I use it on my old Samsung slate 7, which is just not up to the task even though it has quite beefy hardware. The old Wacom digitizer layer is not that good, or it could be bad drivers... anyway, it is hard to get clean, smooth lines on it.
Not much of a problem with Manga Studio thanks to the pen stroke stabilizing. Allows me to work quite accuratly and in detail even on this less than optimal machine.
EDIT:
Just checked current prices on Wacom companion and Surface tablets... if you pick a surface pro 3 with similar hardware to the lower specced Wacom tablet (8G RAM, 256G SSD, i7 CPU), the price is actually not that far apart anymore.... the wacom device is only about 10-20% more expensive (altough you do get older hardware, meaning less battery life, with it).
The surface Pro gives you lower specced options which start at roughly half the price of the wacom companion, and you can get a surface 2 for 50% off nowadays (which again offers 8G RAM and 256G SSD, but just an older i5 CPU) which makes it even cheaper than the cheapest Surface 3 option.
So if you can put all that horsepower to good use and think about a highend device, the surface will not really be that much cheaper.... its nTrig vs wacom digitizer, 12" crisper glossy 3:2 display vs 13" matte less crisp (still Full HD though) 16:9 display, newer CPU with longer usage time vs older CPU with shorter, smaller and less heavy form factor without physical buttons vs heavier bigger with physical express keys. Price difference is small at this point.
Though the type cover, while pricey, is a very nice addon that Wacom should include for their next version.
Speaking about newer versions, word has it that Wacom might release newer companion versions in 2015. So you might want to wait some months if you want to get a companion, to either snatch an improved version with better battery life, or get the old version at a further reduced price.
