I do all my own decorating and I'm quite good now!
Preparation is the key. Fill in any holes/scrathes with polyfiller and then sand any bumps etc until you havea nice smooth surface. Painting over white is easy.
As PP have said watch videos to see how to cut in (paint the edges) and then use a roller for the middle.
Sand any painted woodwork well to make a surface for the paint to catch on, and in high traffic places I would lightly sand between coats. (and do 3 coats)
I don't use masking tape. That is because I have a steady hand cutting in. If you do use masking tape splash out on frog tape, cheap masking tape is hopeless. Before you take it off afterwards run a very sharp knife along the edge of the tape to cut through any paint that is on the tape and the wall.
Be prepared to do 2 good coats, or, if it is a strong colour change, even 3 coats. The second and third coats it is harder to see where you are going, so be systematic as you work round. Don't believe any paint that says one coat is enough, it will look much better with 2.
Avoid coming back and catching the edge of paint which has started to dry, as it will mess it up. I tend to do the whole room cutting in and then go back to the beginning to paint, as the cut in edge is dry by then. the other way is to do both as you move round, 1 m of edge and then the middle etc.
choosing paint:
I made some big mistakes when I first started painting. Any shiny paint (which includes most bathroom paint and kitchen paint) anything called satin finish etc will show the uneveness and bumps of the walls more than matt. I tend to use semi shiny on doors and skirting boards (eggshell finish) and matt on walls, but I think nowadays I would lean towards matt on everything.
BUT if you buy basic matt wall paint, you can't clean it. Look for one of the ones that is designed for places like hallways. Washable/durable/endurance/scrubbable etc etc.
B&Q paints from their mix-your-own colours do good extra durable matt finish (be careful with their off the shelf colours they are not so good).
This paint is more expensive, but you can literally get flash and a scrubbing sponge and clean it. That means it lasts so much longer, and still looks good.
Any if it all goes wrong?
Well just paint over it!