It's helpful to do the freebie games that the site has. Not only those, but the quests too.

Most people get rich by either selling their valuable names or high colored pets, or selliing their rare pets (like CSers, seasonals, etc).

Some people get their TU income by doing graphically inclined things, like drawing art, or making banners, or creating profiles for people.

A lot of people actually get rich from stocks. Stocks is sort of a trial and error for beginners. You kind of have to watch the patterns of the share values. Obviously, the high shares (the ones above 200) are good to invest in when their shares are low. There's some stocks that never reach 100. Stocks are like a long term type of thing if you want to get millions from it. You have to buy shares at least everyday, little by little if you have to. (If you're confused about stocks, I'm sure someone else can explain it better than me. D: )

Others make some TU by selling their hatches, searching for eggs or names for people, etc.

There's tons of ways but you need to find out what works best. xD 

Playing stocks is actually a good idea to make lots of Tu. As long as you invest in the right stocks (such as BAK, FAS, FRE, SQU, TOY, etc.) while they're low (below 100 since they're good stocks), they will go up to palces like the 300s sometime, even if you have to wait a while. There's a good guide to stocks linked in this thread here: http://www.rescreatu.com/forum/cat/general/general/a-guide-to-stocks/~page/1/. Also, the stocks like CAN seem to be good investments because they're so low, but the problem is, they never really go up. I got all this info from the guide above. xD I'm definitely not the best at stocks, since I just started using them around July, but I've already made at least 20-30 mil from them. Finally, if you need any additional help, you can send me an Rmail and I can give you some advice (although, again, I'm not the best) or you can go on the chatbox and ask. There are usually some expert stock players on there. ^^

Another good way to earn Tu is by selling hatches, like Jenny said. You can almost always find someone willing to buy your hatches for 300K on the CB. That's how I got the money to play stocks. 

The last way I earn my Tu is by playing the flash games. When you first start out, they may not give you a lot of Tu, since you're not that great with the game yet. But, like everything, practice makes perfect. I play Blast Off Bunnies, Birds on a Wire, and Cosmic Commander (sometimes). If you get enough points, they will give you 25K, and since you can play them each 3 times a day, you should be able to earn at least 100K a day from them. ^^ They are a little time-consuming, though.