The main gateway is Aviemore, which has a railway station on the Inverness-Edinburgh mainline — journey time is around 40 minutes from Inverness and three hours from Edinburgh (direct, no change required). The Caledonian Sleeper from London Euston to Inverness also stops at Aviemore, arriving in the early morning. Trains run several times daily.
By car, Aviemore is on the A9, the main road running from Perth to Inverness through the Highlands. From Edinburgh it's about two and a half hours; from Glasgow slightly longer. The road is one of the busiest in the Highlands and carries heavy lorry traffic — allow extra time in summer, when tourist traffic adds to the load.
Inverness Airport is the nearest commercial airport, about 40 minutes by taxi or bus from Aviemore. It has connections to London, Edinburgh, Amsterdam, and a few other European destinations. Aberdeen Airport is a slightly longer drive (about an hour and a half) but has more frequent connections.