Maybe you remember when, thanks to The Wall Street Journal, everyone thought the SEC was going to meet on May 7th to decide whether or not Ethereum (ETH) was a security. If you had taken up a large position in ETH prior to the 7th, you would have been disappointed when the price fell from ~$793 USD to ~$743 USD amidst news that there actually was no meeting after all.
A “manageable amount” is obviously subjective and will vary for each person based on things such as time available to dedicate to trading. Feel free to do your own research to find the right exchange for you. I tend to value user experience of an exchange over the amount of coins on it. Ultimately, what exchanges you use is going to depend on your own personal preferences. GDAX and Poloniex will provide sufficient resources needed to be a successful trader, so they are definitely a good place to start.
This learning program will make you completely self-sufficient. There will be no need for you to pay for another course or to subscribe to different trading groups and live trade rooms. There is a ton of groups and individuals out there who are selling or giving away trade signals on their sites, Twitter, forums, etc. All these signals and tips can be useless or even very harmful, if you don't know how to do your own research. For instance, a fresh tweet saying it's time to buy a certain altcoin might not only be old news. There's also a chance it was written because the author wants to actually dump the coin, and he needs people to buy and raise its value at the same time he is selling it.
Don’t be greedy. No one ever lost money taking a profit. As a coin begins to grow, the greed inside us grows along with it. If a coin increases by 30%, why not consider taking profit? Even if goals are set to 40% or 50%, you should at least pull out some of the profit on the way up in case a coin doesn’t reach the goal. If you wait too long or try to get out at a higher point, you risk losing profit you already earned or even turning that profit into a loss. Get into the habit of taking profits and scouting for re-entry if you want to continue reaping potential profits.