Let's see...
It looks to me like Spodumain, Bistot, and Crokite are good targets. Now, these are all found in large amounts in low/null-sec space?
Should be. Not sure where, and every system is different. In general, the lower the security, the more valuable the minerals.
Scratch that, I'm going to be targeting Hedbergite and Gneiss. Which one is usually bought/sold in the largest amounts on the market?
I'm a long time lurker who has registered just to tell you not to mine. Seriously, don't mine. And somewhere above they told you not to mine veld, which is wrong. Still the most profitable ore in hisec but plag (or pyrox? idk) is rapidly spiking due to noc prices. Hed and gneiss are lowsec/nullsec and you will die. When you are not dieing, you will not earn enough to recoup the costs of the dieing.
Really, don't mine. It will ruin the game for you. I think you can make upwards of 15m/hr mining veld in hisec with a hulk (which you won't be able to do for several months, a lot of training). At this, peak mining rate, you will still need to mine for about 35 hours straight to make enough for just 1 plex (30 days of game time). You will find out just how boring mining is, requiring just enough attention to not be an AFK activity, but so little attention that you won't want to actually watch it.
Your best bet is to join a corp that does hisec incursions and train up to fly what they want from you. From what I understand 100m/hr is actually fesable. Not to familiar. Trading can be great too. I went from 100m to 2b in about a month and a half doing simple trading runs. This requires an alt to check prices but I guess there are programs out there that make eve-central usable and can eliminate the alt requirment. With trading, keep as much cash in play as possible. Money in your wallet isn't earning more money.
Seriously, don't mine. I mine but only as a side activity while watching a movie on the second monitor and only to use the minerals in my own manufacturing processes. I hate hauling trit.
There are also a lot of restrictions on what a trial account can and cannot train. Your money making potential will be serverly limited whilst on trial status.
Source: I've been playing eve since 2003 under this username.