No MktMaven here, but I have been self directed with my portfolio for the past 3 decades...
Whether it's $500 or $5000 or $50,000, investing/trading with 'fun money' is fun, and if one can stay out of the emotions involved, it can be a very good learning process.
So, I won't say don't throw $500 or some affordable dollar number at ETrade account, et al, and give it a try. I would avoid day trading, as even on ETrade the costs really chip away. And, investing/taking a flyer/learning is not about wringing your hands every afternoon at 2:30 PM, as to whether you want to 'hold position' over night, or not.
Your four stocks you tossed up are interesting, though I would take a Pasadena on SGLB, as the people that have made any real dough on penny stocks are similar in number to those that make money in Vegas, or with the trotters.
The two 3D companies are interesting, though they would have been a lot more fun back in Jan '13, as they have appreciated a ton this year; outlook? No fookin idea, but 3D isn't going away, but picking the players is tough at this time.
Back at the Investment Ranch, that is the underlying problem: picking stocks.
Most of us can't do it very well, including the overpaid mutual fund 'managers', imo.
I pick some for fun, (with that 'fun money' I ref'd above), but I select most for my real portfolio because I feel a certain stock(s) offer considerable value and potential price appreciation
over time, and pay some dividends now, and in the future. At my age, I am not rolling the dice, or betting my houses.
I have a substantial taxable account and a large IRA, (non-taxable until I must start withdrawals at age 70 1/2), and they are full of individual stocks, a few mutual funds, a lot of cash/cash equivalents, a few bonds, (bonds suck, imo, except for a few that are still paying 5-6% and haven't been 'called'), and a lot of Index Funds.
Index Funds are low maintenance cost, and trail/track nearly any kind of mkt index one wants. ETFs are the thrill of the past couple years, but so what...
Vanguard or similar, Index Funds, kick azz over time. I don't have a lot of time left, per se, but you younger guys do. Unless one thinks the world is coming to an end, wait for a strong pull back, and plow some dough into Dow, Russ 2000, NAZ, et al, via some very low cost Index Fund. Very few Mutual Funds beat the Indices Funds in any time frame comparo, and esp over 'time'.
Not going to lecture on what/how to buy, but I appreciate your initiative: take the $500 or a thou and go have some fun. Learn about Stops, trailing stop losses, sell points, how to read a time price chart, balance sheet, Free Cash Flow, et al. Those kinds of buzzwords and a basic understanding of how a company operates, will save you dough if one/some of your company picks or the Mkt takes a dump, and those terms will force you to learn how these things work, and work for you. Stops of some kind, as well as sell points, also force one to keep the fooking emotion out of the equation, as well as reducing one's greed, imo.
DO NOT BUY ON MARGIN...don't even think about it.
And, if you have a job that offers 401k, start packing as much as you can afford/the company allows: there will be little SS dough when you kiddies get to be my age.
Spend more wisely, save, save, save...regardless of your young age(s), and you might get to retire in your early 50s as I did, 14 years ago.
Investing 101, class dismissed.
GL, mD