If i make less than 400$ for my small business, do I just not file taxes for it? Or do I still file but put the 300$ i made? I dont want to miss filing and be fined

you report the $300.    it's to late to either prepare or e-file your return online.  you must purchase the desktop version for  Mac or PC.  if you need to file state then you need to buy that too or buy a version that includes one state return.      you mention $300, but if that's your only income, you may not required to file a federal return - see below.  state law might be different

see the following link to see if you have to file page 10 unless you are a dependent on another taxpayer's return then page 11   https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

even if your income is below the amounts listed you still have to file if

Chart C—Other Situations When You Must File
You must file a return if any of the seven conditions below apply for 2018.
1. You owe any special taxes, including any of the following.
a. Alternative minimum tax.
b. Additional tax on a qualified plan, including an individual retirement arrangement (IRA), or other tax-favored account.
But if you are filing a return only because you owe this tax, you can file Form 5329 by itself.
c. Household employment taxes. But if you are filing a return only because you owe this tax, you can file Schedule H by itself.
d. Social security and Medicare tax on tips you didn't report to your employer or on wages you received from an employer who didn't withhold these taxes.
e. Write-in taxes, including uncollected social security and Medicare or RRTA tax on tips you reported to your employer or on group-term life insurance and additional taxes on health savings accounts. See the instructions for Schedule 4, line 62.
f. Recapture taxes. See the instructions for line 11a and Schedule 4, lines 60b and 62.
2. You (or your spouse, if filing jointly) received health savings account, Archer MSA, or Medicare Advantage MSA distributions.
3. You had wages of $108.28 or more from a church or qualified church-controlled organization that is exempt from employer social security and Medicare taxes.
5. Advance payments of the premium tax credit were made for you, your spouse, or a dependent who enrolled in coverage through the Marketplace. You or whoever enrolled you should have received Form(s) 1095-A showing the amount of the advance payments.
6. Advance payments of the health coverage tax credit were made for you, your spouse, or a dependent. You or whoever enrolled you should have received Form(s) 1099-H showing the amount of the advance payments.
7. You are required to include amounts in income under section 965 or you have a net tax liability under section 965 that you are paying in installments under section 965(h) or deferred by making an election under section 965(i)