Llanberis is easily reachable by bus. Sherpa busses (prefixed S) from Betws-y-Coed is the most reliable possibility, but Sherpa bus from Porthmadog or local busses from Caernarfon are other alternatives.
Bangor is the closest station to Llanberis. Hiring a cab from there costs around £20. Caernarfon is closer, but the only train to Caernarfon is the narrow-gauge heritage railway Welsh Highland Railway from Porthmadog - and if in Porthmadog it is easier to catch a direct bus to Llanberis.
Llanberis is on the A4086 Caernarfon to Betwys-y-Coed road. The B4547 branches off just west of town for Bangor, Conwy and the A55.
There are large car parks all along the A4086 road inside Llanberis, with the price rising steadily as you get closer to the Snowdon Mountain Railway. The closest car park costs £6 per day (and no discount for shorter stay), 100m up the road it is £4 per day and further west beyond the Electric Mountain Visitor Center it is £2 per day.
On foot easily. You may also want to take a ride on Llanberis Lake Railway  (£6.00)
There are several B&Bs and hotels in Llanberis.
Caernarfon - Bus number 88 goes directly to Caernarfon city center. Can be caught from Llanberis Maes Padarn