I've never been designing device from scratch, and I'm also just first grade student. Anyway I could imagine how this might look for small company or single person:
1) Decide what do you want to build-up. Easiest todo is custom dev-board, it can be always redesigned and packed into tablet case. The hardest to-do is mobile phone, and it's nearly impossible to create such thing due to high level of embedding everything, and need to sign pretty serious agreements with RF CPU (and other things like transceivers, antennas, duplexers) supplier like Infineon or Qualcomm.
2) Think what main components you'll need, like LPDDR, SoC (CPU), PMIC (SoC manufacturer usually recommend PMICs to be used and provide reference board schemas for using both), battery fuel gauge, charging controller (both might be built into PMIC, depends on model), screen+touchscreen (there are dozens of such, one might want to decide its size already, but in case of dev-board like build it usually can be replaced by some smaller/bigger with small HW modifications or without modifications at all), sensors like gyro, compass, pressure, light, whatever.
3) Search through suppliers websites and decide what models of ICs you want to use (I'd pick only open hardware), order engineering samples and get reference schemas, rather start from SoC(OMAP4460 for eg.)+PMIC pair, then decide about the rest.
4) Don't forget about extension slots like USB ports, DC supply, serial converters, whatsoever.
5) Start designing PCB board. IMO it's impossible for begginer to project any usable PCB for embedded system, I'm begginer and I'm failing with simplest boost HF DC/DC converters (like 10-20 parts on board), while such board would have thousands of elements on it, and multi layer board to fit it everything in some rational size.
6) Find company that will make prototype for you - they should make board + solder all the components you provide them - one with no professional (and very, very expensive) soldering stations is not able to solder BGA components at home.
7) Test it out.
Relatively, assuming that main components are free engineering samples, this might be not so money-expensive way to create some useful stuff. But for sure it's very, very time expensive, and begginer alone will nearly for sure fail.
I just re-read my post and figured it might be pretty demotivating. I'm not saying it's impossible, but I'd suggest you to start from something easier - ARM dev board is the thing you need. As Quiggers stated above.