This is for those in the US who have to comply with American tax law. I understand the UK just embraced the same law, but don't know whether it applies to 2018 or not.
If you're in the US, you need to file a form 8949. That's basically a profit and loss statement with every single trade on it, including the US dollar value at the time of each leg of the trade even if you were just going from crypto to crypto.
E.g. every time you sell BTC/ETH to buy an altcoin, the US govt. considers that a taxable event.
Pain in the ass, right?
The Good News
Though many of us hated that the market kept falling all year for 2018 AND also think this tax law to be ridiculous, the good news is that if you bought ANY altcoins after January, you probably have losses that can offset your income for the year, reducing your taxes.
The amount you can deduct in any one year tops out at $3,000. If you lost more than that (according to this law, not the way you may think of losses yourself) you can carry that forward into future years at the rate of $3,000 per year.
So if you have crypto gains in 2019, if you had enough trades into a falling market (relative to USD) in 2018, it will be as if you gained up to $3,000 less in 2019. And that's not just against crypto gains. It can also offset your gains in selling gold, silver, etc.
(I think sometimes gold/silver may count as collectibles sales, which I don't know whether form 8949 losses can offset those gains. But I think in the end you get the deductions on your overall tax liability, including even regular income. Ask a tax professional for details on your situation.)
What to Use
I decided to use Bear.Tax because it was cheapest and also had an API integration with Binance. I also took a look at TokenTax.co which I might have used to have them completely file for me, not just give me the profit/loss form, if I wasn't needing to use my tax accountant to do some more complicated tax accounting for other stuff I had going on in 2018.
It took me a total of 20 minutes to do everything, which included the time it took me to wait to get a coupon code for 10% off in exchange for reviewing BearTax on their Facebook page and tweeting about them. I actually wound up getting 20% off, so that may be because the 10% end of year discount hasn't been turned off yet. I don't expect that will continue indefinitely.
There are others out there too, but these are the only ones I've investigated enough to say anything about them.
I'm not a tax professional, so can't advise you to do X, Y or Z, but I can tell you that you are affected by these taxes and need to submit this form if you're an American who sold any crypto (even to buy other crypto) in 2018. If you need more hand-holding than one of these crypto tax sites offers you, then definitely do talk to a tax accountant.