The $25k limit in the US is for those who do the "pattern day trading" - buying and selling the same security in the same day on 3 or more occasions in a 5 day period. There's no such limit if you're not doing it that way, and I personally speak from experience when I say you probably shouldn't "pattern day trade". I'd say start doing some homework and start investing!
I think that it is safe to say that you should have something like 30k. Another thing to ensure is that this is money that you can lose because most of the time when you are starting out is going to be used to learn the ropes.
The real answer is just in how often you think you are going to be selling. If you want to invest a few hundred every month, you can easily do that through funds... if you are going to buy and hold a stock, you should probably shoot for $500 per order. If you plan on buying and selling within the same year, then you probab ly shouldn´t make many buy orders with less than $1k or $2k. The more frequently you buy and sell, the bigger the amount you need to invest each time you buy