Regarding your private keys, can either keep them on a software wallet, or on a hardware wallet. The software wallet is the easiest way for beginners, but it implies that you have a computer that is properly secured (strong password, antivirus etc.) and that you do backups of your data. Remember, if you lose your private keys, the money is gone forever. As you get into cryptocurrencies more seriously, you will inevitably look for a hardware wallet, which is a dedicated device resistant to hacking where you can store your private keys (or to say it in an easy way, your crypto assets/invested money). I have already written an article on a hardware wallet, the TREZOR (see here). The most common wallets are the TREZOR and the Ledger Nano S. The TREZOR has a new model being sold from January 2018 (Model T) which will support much more currencies than the current TREZOR. The Ledger Nano S has more integrations currently, but I would recommend to wait for the TREZOR T to judge properly. Those devices usually cost around 100 USD, so once you have more than 100 USD to protect it starts making sense to get one.
While most online brokers do not offer international trading, some do. Nearly every broker supports trading American depositary receipts (ADRs), which offers US investors an easy, simple way to invest in foreign companies. However, if you want to buy physical shares of an international company, then you need to do your research. Interactive Brokers is the leader in this space but is built for professionals. For casual investing, both Fidelity and Charles Schwab offer international stock trading.
Dollar-cost average: This sounds complicated, but it’s not. Dollar-cost averaging means investing a set amount of money at regular intervals, such as once per week or month. That set amount buys more shares when the stock price goes down and fewer shares when it rises, but overall, it evens out the average price you pay. Some online brokerage firms let investors set up an automated investing schedule.