achievable ways that i'm sustainable
The first way I make these achievable is by far the most important: Don't make anything a hard rule - just a goal and a habit.
I purchase most of my clothes secondhand. They aren't all from thrift stores either - there are tons of vintage, resale, secondhand, etc. stores out there! I also make a point to not be too strict about this. It's okay to buy new clothes when it makes sense to!
I rarely replace things unless they are broken or no longer functional. Obviously, I make exceptions sometimes, but I try to use what I have until it can't be used anymore. A great example of this is that I wanted to get new bathmats - so I put my old ones elsewhere in the apartment rather than getting rid of them.
Before I donate or throw away usable things, I try to give them away, sell them, or repurpose them. Most places that get donations are incredibly overrun by the volume of donations and much of what's donating ends up in the garbage or in a mountain of clothing somewhere, so I try to leave donation as a last resort. I offer things to friends and family, sell them online, or try to find another use for them before I resort to donation. Also, be sure to research the organizations you donate to - some of them are no good at all! It's also a good idea to do your research and try to donate things to places I know they'll get used rather than somewhere like Goodwill.
I clean my own water rather than buying water bottles. Where I live, tap water isn't drinkable. However, rather than spending valuable money on wasteful bottles of water, I made a one-time purchase for appropriately heavy-duty water filters and/or a way to boil water before drinking it. If that's not achievable for you, it's always cheaper and less wasteful to buy your water in bulk.
I eat at home or at local restaurants as much as possible. Of course, fast food is unavoidable sometimes. But I find that it's fairly easy to eat at home or at a local restaurant once you get into the groove of it - not all home-cooked meals are complicated or time-consuming, and most local restaurants can do take-out very easily (and often, pretty cheaply)!
I bring my own bags not only to the grocery store, but everywhere I go. I keep my wallet and epi-pens in a reasonably large tote bag, so when I grab something at the convenience store or run into a small shop, I don't generally need to get a bag. When it comes to the grocery store, I always take my own bags - usually one refrigerator bag and one regular bag - and also bring my net produce bags (from Ikea, about $3 USD each). This helps me avoid unnecessary plastic use and keep trash out of my local environment.
Since I live in a large city, I drive as little as possible. By that, I mean that I essentially only drive when I commute to school, as my school is not in the same city and is not currently accessible by public transportation. Walking and biking are great ways to get around and stay healthy, and public transportation is a fantastic option when it's too far to walk or bike. I'd encourage everyone to try out walking, biking, and transit - no matter where you live! It might surprise you how achievable, cutting down on car usage is.