Investing in Bitcoin is most easily and directly done by buying BTC units however you can then storing them offline. If you are investing more than $5k - $10k, you might want to divert a small amount to developers of open source projects for Bitcoin or to tipping. If you want more leverage, you can invest a similarly small amount in one or two of the altcoins since these tend to rise and fall more than Bitcoin in the case of a genuine, new money bubble.