How to invest is easy; with a little research, you should find that putting most of your assets in shares, essentially on auto-pilot, applying dollar-cost averaging to avery broadly-based index-tracking ETF like the MSCI World will give you a good (evidence-based!) risk-reward pay-off. Buying a recent edition of Security Analysis (Graham & Dodd) will turn out to be money well spent - not a light read, but you need to know the research, know the why of what you are doing (also, just reading that monstrous tome will noticeably improve upper-body musculature).
Being an investor is difficult in a way I am sure many posters here will agree with: you will likely find that learning the necessary emotional discipline , or more elegantly, sang froid is the hardest part; which is to say, to learn to be happy with doing the rational, evidence-based thing. Right now, as an indebted person, you pay off a nice slice of your debt every month and make, more or less, a guaranteed return, and find yourself the paid-up owner of an ever-larger fraction of the house around you. You need to exert continual self-discipline to keep to what I am sure is a very accelerated repayment schedule, but you have the great advantage of a very reliable carrot.
Soon, however, you will find yourself accumulating significant liquid assets, watching the sums mount steadily through five digits and into six, zigging towards the great threshold of FI. Now, you have new problem: at a click of a mouse, you can send your money off to moulder on deposit, to do a little better in the bond market, to chase some shiny commodity, to follow a hot share tip, or into the capable hands of some professional money manager. I personally had no emotional difficulty doing this when the total amount was still in the low to middling five digit range. I was completely comfortable with risk, and had far too much of it (I was a stock picker!). When I found myself in the land of six digits, things became much more difficult. The amounts were just too large; the upside fabulously tempting, but the prospective losses terrifying. I went into a funk, and spent the better part of a year in cash. I felt terrible; I now knew exactly what I should do with my money, but couldn't bear to proceed.
In the end, I realised that the only way to get moving was for the rational part of my brain to play some tricks and even to compromise a little. I would bribe my emotional side with a relatively modest investment in a very exciting bank that was making incredible returns for some close friends, and the rest I would put into sensible and boring etfs. I did at least try to cast a broad net with my index trackers. Another existing risky investment was too exciting to abandon - but I was able to convince myself to reduce my position and move at least some of that risky money into a more sensible place.
Well, along came the crash - and that thrilling bank went to zero, while the etfs dipped, then grew back. It was painful - but, at last, my emotional side came to love those boring index-trackers. I also learned to feel the truth of something I had read previously; that checking too often the current market value of your holdings is a really bad idea. If returns are flat, you will be tempted to put your money in some hot new field; if down, you will feel poorer and foolish for the "loss" (which, of course, is still a purely theoretical one - you've not sold yet) - and maybe tempted by "safer" (i.e. more inflation-vulnerable) assets. If up, you will feel very rich and clever, which is also dangerous. You will become more confident, less cautious - and much less inclined to save (this happened to me). It is emotionally difficult to save when your investments are piling up; I had this problem myself when Mr. Market was giving me over-generous valuations, and, yes, I saved less. It just didn't feel necessary - even though I knew that Mr. Market is a bit of a manic-depressive, and I should not believe too much in the prices of any given day.
I'll end with a story about some retirees in Florida, sitting around a pool, discussing the paths that had led them to FI. The topic turned to return on investment - who had achieved what rate of return on their assets? The bragging moved around the pool, until it hit the group's philosopher. "I've got no idea", he said. "I don't even know whether I beat the market... but I made enough to get here.".
And that is the salient point. We aren't trying to get rich quick; we are trying to get to FI, reliably but without undue delay. Remember that the exact rate of return you get is not that important. Don't be greedy, and don't try to be fearful. Just try for a market return with the lowest possible transaction costs, and stick with it. It will take a while to learn to ignore Mr. Market's mood swings, but it is a trick you can learn.
