The busstation is a bit out of town, but there are collectivos the center.
There are frequent busses to Pasto and futher north. Collectivos will bring go to Rumichaca for the bordercrossing if you go to Ecuador. The bordercrossig is quite easy and straightforward. The Colombian DAS-office is on one site of the bridge and the Ecuadorian office in on walking distance on the other side. From there are collectivos to Tulcan. Don't sell your dollars on the border, the exchange rate is bad.