I definitely don't trade on the stock market but I did try it, believe it or not I did try I also tried forex, lost quite a bit of money
little did I know back then that it was so badly rigged. What I'm doing is I'm working on Bitcoin related stuff, so things like cloud mining so that I can gain a relatively stable income and what I'm doing is saving up so I can increase my hashing without spending anymore money. You won't get immediate ROI ( Return on investment ) on some of this but over time you will gain more and more over time and you can put that into more riskier investments at least that's what I've got planned.
If you can genuinely get $20 a month then you should search for non-scammy passive investments, you have a job, so that's something, but maybe if you can find a higher paying job that might be more worthwhile even if you've got some spending money left over each week you can re-invest that into income when you'd otherwise try and save it etc. because your money is going to get constantly devalued anyway.
As much or as little as you can afford to invest. Bitcoin isn't a magical get rich quick scheme, but a lot of us can see the potential in it for investment but nobody can tell you how much btc will be worth in the future as it is all baseless speculation.
And Rampton just said it best, absolutely, don't be stupid with your money, can't count the number of times we've had people coming in here making threads about how they lost all their life savings and so on or are in massive amounts of debt because they got stupid, the great thing is about Bitcoin is it's a genuine open market so you can go in with just $5 a week if you want just make sure it doesn't cut into your living standards unless you're ditching a game or something you'd normally buy
Whatever you do don't let yourself get further in debt, that's for damn sure, you're going to have to do a lot of maths and double check everything, be pessimistic, signature campaigns are also another way of getting some extra income as long as you don't just spam as you can see that's a bit of what I'm doing.
Edit: Oh and the best thing about getting any income in Bitcoin is if the price shoots up you get a discount on stuff
sometimes a major one and that can make all the difference.