Riga is the capital of Latvia.
If you want to get towards the south it is best to get out of the city center first. Take tram number 10 towards ??? and get out at ???. You will see a gas station just around the corner. Asking people there is usually more successful than standing at the street.
Another possibility, from the city centre, taking a microbus to Kekava until the last stop and in Kekava you take the main road in the direction of Bauska.
Good place to stay but no gas-station: Take bus nr.23 (opposite of Stockmanns) towards Balozi. Before the bus turns right to Balozi - go off and you've got a traffic-light and a bus stop for stopping cars - everyone's going towards Bauska!!
Cross the main bridge southwards, you can walk, but possibly there is a nice bus or minibus you can take. You keep on going until there is a crossroad with the A8 highway to Jelgava.
Take Tram nr.6 from city centre (Katedrale) to the final stop in Jugla - there you find the road to the north including gas station for asking, traffic-light and McDonald's (toilet). Note, that about 750 metres farther from the traffic light is a nice big layby where cars can stop. If you're heading for Estonia have an EST sign - makes it all go a lot quicker.