If you're going to start investing, you need to accrue some working capital - that means you need to earn some gold doing other things so you have gold to buy stuff in the MP.
If you have lots of real world money, it's easy to do: buy cash shop items with real world money and sell them in the MP for Gaia Gold.
If you don't, or you want to do this all on Gaia's dime, you need to earn gold in other ways. One of the best ways to do this is playing Booty Grab. Check out
the aquarium forum for directions and endless threads of potential tanks to play it on.
Never pass up an opportunity to earn easy gold, even if it's just a little. For example: you get 60 gold for the first posting every day in your Gaia Journal. That's 21900 gold over a year. Posting in forums. Voting in polls. These activities have diminishing returns, but they take almost no time every day and it adds up.
And be sure to collect your Daily
Crap Chance stuff every day. Again, it's mostly junk, but you can sell it back to the stores for gold or in the MP or whatever and it adds up. Same with visiting Skittles World or whatever other sponsored area may give you stuff daily. You can always trash the items if they're junk.
The only risk-free investing is buying store items in the MP for under half the store price and selling them back to the stores for half the store price. (If you didn't know you could do this, maybe it's a little bit early to be thinking about investing.) The good news is that this is 100% risk free - you know exactly how much you'll be getting for your item, and it takes about 15 seconds to get it. The bad news is that this is no secret, so you'll be competing with scads of people to purchase these underpriced items. It's good practice in losing, because you'll be doing a lot of that in the MP. We all do.
Any other investing you do involves risk - which means
you may lose gold on it. Never invest more gold in anything than you're willing to tie up in it for awhile. This is why, even if you've got a good amount of working capital, it's always good to keep the extra gold coming in on the side, so if you do end up stuck with something where you have to wait days, weeks, or months for the price to go back up to where you want to sell, you still have gold to keep investing with.
EIs can be a good investment - but a bad evolution or a disappointing finish can send the price into the basement. In general, the more expensive the EI in the Cash Shop, the more likely it is to be a good investment. Because Gaia often gives out last generation lower priced EIs in RIGs and the like, which can put a lot on the market, which can lower the price overall.
Remember that *anything* that is not explicitly stated as limited - like Monthly Collectables - can and will be sold or given out or whatever by Gaia again. Take this under advisement when you're investing for longer term.
Speaking in general, which means of course there are lots of exceptions, the longer you hold on to an item, the more it will be worth when you sell it. However, the longer you hold on to it, the longer you have some of your working capital tied up in it, which means you can't use that gold to take advantage of other potential oppotunities. Of course, once you have lots and lots of working capital, you don't have to worry about this so much. But assuming you don't, you need to make a bunch of decisions about profit now and the ability to buy and sell other things with that gold or make more profit later. And only you can make that decision for yourself.
And never forget the 2% tax in the MP. It makes a difference, especially with higher priced items. You may want to look into the Exchange for selling those, but someone else will have to give advice on that because I don't like dealing with it.
Which comes to perhaps the most important advice I have:
1. Don't do anything you're not comfortable with. Different people have different tolerances for risk and uncertainty, and a counter on a game (which is all that gold really is) is not worth beating yourself up about or losing sleep over or fretting about. It's just not worth it. Find your own style and stick to it and don't worry that other people do it differently. If you're making gold, it's working for you.
2. Don't violate the Gaia TOS. Don't exploit glitches. Being banned not only sucks, it also means you've just lost everything you've been working towards. The risk is too high for the gains you might make. It's just not worth it.