Switzerland has three main international airports: Zurich (ZRH), Geneva (GVA), and EuroAirport Basel-Mulhouse-Freiburg (BSL), which straddles the French-Swiss border. Zurich is the busiest, with direct long-haul connections to North America, Asia, and the Middle East. Geneva is the gateway to western Switzerland and the Alps, with strong European connections including several low-cost carriers. Basel is smaller and handles mostly European flights, but it is very convenient if you are heading to Basel itself or the surrounding region.
Swiss International Air Lines (SWISS) is the national carrier and operates from Zurich as its main hub.
By train from within Europe, Switzerland is exceptionally well connected. Direct trains run from Paris (about 3.5 hours to Basel or Geneva via TGV Lyria), Frankfurt (about 3 hours to Basel), Milan (about 3.5 hours to Zurich), and Vienna (about 8 hours). Eurostar connections via London are possible with one change. For most European visitors, arriving by rail is more convenient and often faster than flying once you factor in airport time. Check schedules at sbb.ch.
By car, Switzerland is easy to reach on the European motorway network. Note that all motorways in Switzerland require a Vignette sticker (currently CHF 40 per year), sold at border crossings, petrol stations, and post offices. Without one you face a fine.
There is no meaningful long-distance bus travel into Switzerland worth considering if you have other options, though Flixbus does operate some routes.