For US tax purposes tips must be included in earned income. This is not an option you choose. This means you must pay Social Security and Medicare taxes on them (for SS only up to the yearly cap, but that doesn't matter for you) and you get credits accordingly. You must also include them in computing income tax, but if your total income is under the threshhold amount (raised to $12k for single this year by TCJA) your income tax is zero. And you must pay Additional Medicare Tax on earned income over $200k single, but that isn't an issue for you.
See IRS publication 531 Reporting Tip Income (version on website is still for 2017; most pubs and forms for 2018 won't come out until about November, but only some details will change). The normal method is that you report your tips to your employer at least monthly; they withhold as needed (computed the same way as for wages), and next January they include this amount in your W-2 which is filed with SSA and you get credits for the year accordingly. (You will likely file your income tax return at almost the same time, but it doesn't matter if you don't.)
If you fail to report to your employer, or they fail to withhold the needed amounts (per your W-2), there are specific sections on that. These you do add to your income tax return, and IRS relays the relevant information to SSA. (This is similar to self-employed people, who file Schedule SE with their income tax return and IRS relays their SS and Medicare taxes and earnings to SSA.)