Cardiff is the capital of Wales.
When you're coming to Cardiff and get dropped off at a motorway junction on the M4 it's best to go for J32 then walk about half a mile down the A4054 towards Whitchurch and you'll find a train station where you can get to the centre of Cardiff for about £2.
Direction: All directions
M4 Motorway East via A48
Walk out of Cardiff along Newport Road, or take the North-East bound bus to Penylan. There is a nice spot by a car shop just down from the bus shop, with a lay-by where cars can easily stop. It's a wide road with busy traffic, so you'll be easily noticeable.
The road will take you to the A48, which leads to the M4 going East. Plenty of people in that part of Cardiff will be going this way to get onto the motorway.
Direction: Swansea, North Wales
M4 Motorway West
Get the number 57 or 58 bus to Pontprennau. Get off at ASDA, walk to the roundabout, turn left, walk up the dual carridgeway to the main roundabout and you'll be able to hitch on the exit roads to the motorway, east or west.