Soil. They are tolerant of a wide range of soils. Any moderately fertile soil will do. Some species actually prefer a leaner soil. None of the common species require fertiliser beyond decent soil and a mulch or compost topdressing.

Water. They aren't exactly drought-tolerant but don't need masses of water. Some species are mediterranean plants that can't stand being waterlogged, but most are very tolerant.

Deadheading. Again, lots of variation. Many species flower in spring a then not again, although a few have different habits. Deadhead to control seeding and keep the plant looking tidy. A number of hybrids and some other cultivars have longer bloom times and may benefit from deadheading, but a number are sterile and don't need to be deadheaded at all.

Other pruning. Many species are big sprawling plants and you might want to shear them back after flowering or in the middle of summer. Others grow as compact mounds and don't need any help from you. I cut most types back when new growth appears in spring but even this isn't completely necessary.

There are many species, so lots of exceptions to these tips. Some geraniums grow best as annuals/biennials, others are summer-dormant, some thrive in sun and some in shade.