All excellent advice from Fordy.
I did one side of my car last year - front jacking point and loads of sill patches, plus front & back bumper mounts. Its a bit of a messy dirty, fiddly job sometimes as your lying under the car trying to weld, but if you take your time its easy enough. Hard bit is sometimes finding decent metal to weld against. And, once you cut your old rotten jacking point off, you may be surprised at how much more rotten everything is than you first thought.
Couple of other things. I found cheapest place for welding wire was ebay - big rolls of the stuff. I used 0.8mm (if i remember rightly) sheet metal for the sills. Quite easy to cut rotten bits out then weld new bits in. Get some underseal / anti rust stuff to spray about too.
Word of warning. Check whats the other side when your welding. You dont want to set fire to carpets or melt wires. Also, make sure you finish all the welding before you apply underseal. I found the underseal I used was mega flamable.... ahem... car was up in flames at one point before I got the hose on it. Luckily only damage done was to my underpants.
I found watching a few youtube videos on welding really helped me understand it. Plus, start off with the welders recommended settings based upon the material you are welding - they arnt usually far off. Then you can play with stuff like the wire speed if need be. Practise makes perfect, so practise a bit with some spare metal. Take your time and its all pretty easy! Good luck.