If your main focus is college, your best option would be to finish high school where you are now and come to the US for college on your own. You can finance your own cost of attendance (including living expenses) with financial aid - scholarships, grants, jobs and student loans.
If you want to bring your parents over, the best time to do that would be after you finish college and have a full-time job.
As a US citizen, you can (in theory) sponsor your parents for a green card. However, probably not right away:
- You must be at least 21 years old.
- You must be domiciled in the US. (Which means you have to move to the US first before you can start the visa process for your mom.)
- Your parent(s) need a financial sponsor. If that’s you, you’d currently need to earn at least ~ $26,000 a year to support a family of 3. (125% of the federal poverty line for your household size.)
You should also find out why your mom had a hard time getting a US visa in the past. Some reasons may be an obstacle for a tourist visa but would not prevent her from getting a family-based green card (e.g. suspected immigration intent). Some reasons would also prevent her from immigrating (e.g. an entry bar due to previous unlawful presence in the country - has your mom ever overstayed a US visa?).
If you email the US consulate that processed your mom’s visa applications, they should be able to give you a brief reason for the visa refusal(s). Likely a reference to the appropriate paragraph in the Immigration and Naturalization Act. You’ll have to look that up to see what it says.
Here’s a full list of the possible reasons for a visa denial: https://travel.state.gov/content/dam/visas/Statistics/AnnualReports/FY2016AnnualReport/FY16AnnualReport-TableXX.pdf
Here’s the official guide to family-sponsored immigration visas: https://travel.state.gov/content/travel/en/us-visas/immigrate/family-immigration/family-based-immigrant-visas.html