I taught myself a bit of MIG welding over a summer. I hadn't been on any course just read up on some sites and then did alot of practice - welder from screwfix, a big reel of wire and luckily my sister had a summer job at the local pub who supplied us with very cheap gas (cant afford argon). I started off with relatively fat metal and worked down to as thin / 'rotten' metal as possible that wouldn't disappear with 100% CO2. (fiber glass and filler always win then)
I need to modify my cross-flow sump next to make it flatter and so give some actual ground clearance and have been told / read i shouldn't MIG it as it will leak..

. has anyone tried MIG brazing - the wire is expensive?
http://www.mig-welding.co.uk/brazing.htm
As for mechanical stuff, Just get a load of manuals (oilier the better ( £2 each from local scrappy)) then the good old 'Haynes' manual translations are usually more than correct:
Haynes: Rotate anticlockwise.
Translation: Clamp with mole grips then beat repeatedly with
hammer .......................anticlockwise.
Haynes: This is a snug fit.
Translation: You will skin your knuckles!........on both hands.
Haynes: This is a tight fit.
Translation: Not a hope in hell matey!
Haynes: As described in Chapter 7...
Translation: That'll teach you not to read through before you start,
now you are looking at scary photos of the inside of a gearbox.
Haynes: Pry...
Translation: Hammer a screwdriver into...
Haynes: Undo...
Translation: Go buy a tin of WD40 (catering size).
Haynes: Retain tiny spring...
Translation: "Jeez what was that, it nearly had my eye out"!
Haynes: Press and rotate to remove bulb...
Translation: OK - that's the glass bit off, now fetch some good
pliers to dig out that pesky bayonet bit.
Haynes: Lightly...
Translation: Start off lightly and build up till the veins on your
forehead are throbbing. Then re-check the manual because this cannot
be 'lightly' what you are doing now.
Haynes: Weekly checks...
Translation: If it isn't broken don't fix it!
Haynes: Routine maintenance...
Translation: If it isn't broken... it's about to be!
Haynes: One spanner rating.
Translation: Your Mum could do this... so how did you manage to
botch it up?
Haynes: Two spanner rating.
Translation: Now you may think that you can do this because two is a
low, tiny, 'widdle number... but you also thought the wiring diagram
was a map of the Tokyo underground (in fact that would have been more
use to you).
Haynes: Three spanner rating.
Translation: But Ducatis are easy to maintain right... right? So
you think three Ducati spanners has got to be like a 'regular bike'
two spanner job.
Haynes: Four spanner rating.
Translation: You aren't seriously considering this are you, you pleb!
Haynes: Five spanner rating.
Translation: OK - but don't expect to ride it afterwards!!!
Haynes: If not, you can fabricate your own special tool like this...
Translation: hahahahahahahahahahahahahahahahahahahahahaha!!!!
Haynes: Compress...
Translation: Squeeze with all your might, jump up and down on, swear
at, throw at the garage wall, then search in the dark corner of the
garage For whilst muttering under your breath.
Haynes: Inspect...
Translation: Squint at it really hard and pretend you know what you
are looking at, then declare in a loud knowing voice to your
wife "Yep, as I thought, it's going to need a new one"!
Haynes: Carefully...
Translation: You are about to cut yourself!
Haynes: Retaining nut...
Translation: Yes, that's it, that big spherical blob of rust.
Haynes: Get an assistant...
Translation: Prepare to humiliate yourself in front of someone you
know.
Haynes: Turning the engine will be easier with the spark plugs
removed.
Translation: However, starting the engine afterwards will be much
harder. Once that sinking pit of your stomach feeling has
subsided, you can start to feel deeply ashamed as you gingerly
refit the spark plugs.
Haynes: Refitting is the reverse sequence to removal.
Translation: But you swear in different places.
Haynes: Prise away plastic locating pegs...
Translation: Snap off...
Haynes: Using a suitable drift...
Translation: The biggest nail in your tool box isn't a suitable drift!
Haynes: Everyday toolkit
Translation: Ensure you have an RAC(AAA) Card & Mobile Phone
Haynes: Apply moderate heat...
Translation: Placing your mouth near it and huffing isn't moderate
heat.
Haynes: Index
Translation: List of all the things in the book except the thing you
want to do!
I find looking on forums / asking questions has been the best way to get past any major problems.
Oh and on a different note: why cant student terrace houses in Sheffield ever have garages.... and what is it with angle grinders that seem to attract kids from far and wide....