Let me jump in on the contests concept.
Forex contests teach the very opposite of what people need to learn. They are all about making the most profit in the least amount of time possible with no regard to risk. The flaw in this is that for every super-profitable winner, there are huge numbers of accounts that blew out. It really comes down to who gets lucky while doing things that no rational person should do with their investment money.
Regarding slippage, yes, that's where the price you click on to open or close a trade isn't quite the price you get. Many brokers have a very bad reputation for large slippage that's always against the client. You can also get requotes that ask you to accept a different price. If you are scalping and get a requote on entry, you can skip the trade. If you get a requote on exit and price is going against you, say goodbye to your profit and hello to a loss.
I'm glad you downloaded a demo copy of MT4. One word of advice on demo accounts. Get a spare gmail account. Use that and a fake name/phone number to sign up for demos. Otherwise, you'll have brokers calling and emailing you while desperately trying to get you to open a live account before you are ready.
Now get in that Space Shuttle and hit the launch button!