For cracks between edges like between walls and ceiling walls and skirting etc. I use decorators filler (also known as decorators caulk) with a gun applicator. This has about 10% flexibility in it so will help prevent future cracking. Available at all decorators merchants.

To open, unscrew the nozzle and cut the end of the filler with a sharp knife. Replace the nozzle then cut that open at a 45 degree angle approximately 15mm up. (it's tapered so the higher the cut the bigger the hole)

To use, run the nozzle down the line of the crack and squeeze gently. Try not to get too much on. Run your finger over the filler to force it in to the cracks then wipe off the excess with a damp cloth. This type of filler does not sand so well so try to get a nice clean smooth finish before leaving to dry.

For holes and cracks on wall/ceilings I use a general purpose light weight filler. These relatively new types of filler are great. They are easy to apply, dry quickly, don't shrink into the hole as they dry and are incredibly easy to sand.
I apply it with a flexible scraper.
You should be able to fill most holes/cracks with one fill. If you're careful you can fill it right to the surface. If you over fill slightly it's easy to sand off any excess with some medium sand paper. e.g.Red devil one time filler

For filling wood you can use a light-weight filler but I prefer to use a two pack wood filler which gives a much tougher finish. A two pack filler comes in "two packs" a large tub of filler and a smaller tube of hardener.

Just mix them together and use straight away as it hardens very quickly. I try not to apply too much as it s not so easy to sand compared to light weight filler, but it still will with a medium grade sand paper. This comes in a natural wood colour so can be useful if you are filling wood prior to varnishing as its blends in quite well. (also available in white). It's a bit smelly so ensure good ventilation.

NB see section on repairs and maintenance for treating/ filling exterior wood which has gone a bit rotten.

Sanding is necessary to create a smooth surface on which to paint, and especially on wood, provides a key which creates better adhesion of the new paint.

Sand paper comes in various grades for various jobs its roughness it indicated by a number. Generally speaking the poorer the surface, or the greater the amount you want to sand off, use a rougher sandpaper.

Walls
If your walls /ceilings have been badly painted previously you'll need to sand off or scrape off any little lumps and bumps. If it's extensive make it much easier with a sanding plate on a pole. Sometimes you won't need to sand the walls at all.
For walls I usually use a 80g (medium) sheet.

Woodwork

For bad paint work I use a 80g (medium) sheet prior to undercoating. Between coats use something a little finer like 180g (fine).

If the paintwork is so very bad that it is peeling or flaking which you often find often with neglected exterior woodwork then you may need to remove the paint entirely. (see section on repairing exterior wood work.)

Mask off any tricky to paint areas. Some rooms are easier than others. In a bedroom the only masking I tend to do is on the floor or carpet around the skirting. It takes a little time to do the properly but makes the painting so much quicker and gives much better results. On carpet I try to press the masking tape right down between the carpet and skirting. Rooms like kitchens may need more areas masked of.

I would recommend you use 2" tape as any thinner is more fiddly to apply and you also risk getting paint where you don't want it.

If you are very unsteady with a brush and feel you need to mask around walls etc. that's fine but make sure you use a "low tack" masking tape otherwise you might well pull off newly painted areas when you remove it. Again I would use at least 2" tape as you get much straighter lines with wide stuff.