Just plant tons of stuff and see what sticks. Generally local plants and those from slightly warmer climes work well (climate change). Once you have structure (something to shade others from sun) your species availability gets far higher. Also, you need biomass which can be as simple as collecting rotting logs, leaves and sticks (preferably with fungi present). The more stuff you have, the better it gets. Then one day you turn around and you have a jungle with fungi, ferns, orchids, mosses, birds, reptiles, arachnids, so many ant species you'd be amazed, and all manner of weird insects. It's awesome. Human trajectory wise we are possibly around peak bio-availability for species right now. That is, while we are losing them in nature the commercial availability has never been so high. It would be reasonable to expect it to drop in future. Therefore if you are lucky enough to have land it's very rewarding to garden: just don't get stuck in the lawn-and-blower, pristine presentation, white picket fence trap.

Key things to worry about: try to avoid placing spiky and poisonous plants where people walk, or highly invasive root systems like figs near pipes and building substructures. Try to pick species that self-limit to a rational height, you don't want to have to hire people to prune your garden when it decides to fall on your house, infrastructure or members of the public. Otherwise just go for it. Don't get discouraged if stuff dies, that's normal.

Thrift methods: Guerilla gardening on public or disused land. Seed and spore can be readily collected and germinated. Plants can often be divided. You can use an outdoor area as a germination zone by adding extra moisture and controlling maximum sunlight exposure. You can go vertical with climbers, many of which have excellent flowers and do not need much earth. If you have a raised balcony or cantilevered deck, you can let plants flow down the exterior, a technique long popular in Vietnam.

If you have particularly bad soil you can grow in pots or learn to graft rootstock, but this is tedious and expensive relative to the alternative of direct sewing and will not result so easily in an integrated ecosystem.

Personal top botanically inspiring places in the US: Brooklyn Botanic Garden, Chicago's Lincoln Conservatory, Missouri Botanic Garden.