Before you buy anything, answer one question: what is the most I am willing to lose on this trade? That number is your risk per trade, and deciding it in advance is what keeps a single bad call from hurting your whole account.
A common guardrail is the 1% rule: risk no more than 1% of your account on any one trade. If your account is 1,000 dollars, the most you let yourself lose on a trade is 10 dollars. Lose that, your stop takes you out, and you live to trade again.
This is not the same as only buying 10 dollars of the coin. Your position can be larger. The 1% is the distance from your entry down to your stop loss, times how much you hold. You size the position so that if the stop is hit, the loss equals your 1%.
Risk per trade is the loss you accept if your stop is hit, not the size of the position. Cap it near 1% of your account and one loss can never sink you.
Tip. With a 1% cap, you can be wrong ten times in a row and still have most of your account. That is the whole point: small, survivable losses buy you enough tries for your edge to show up.