Before adding compression, find a basic volume level for your vocal track, in the context of a mix containing all the instruments. Assess how much dynamic range there is within the vocal track, focusing on any points at which the vocal 'jumps out' of the mix (when the volume has peaked) or 'dips' (when volume is too low in the mix).
Set the threshold level of your compressor. This is the level (in dBs) above which any volume peaks will be compressed. Set the ratio of your compressor. This is the ratio by which any volume peaks above the threshold level will be reduced. Start with a ratio of 2:1.
Set the Attack parameter of your compressor to a fast time (start with 1 millisecond) and the Release to around 0.5 seconds. Listen again to your mix. If you feel unsure about these settings, use the compressor's Auto Attack / Release function if possible. If your compressor has an adjustable 'knee', set this to a 'soft' level, rather than 'hard.' This means the level of compression you have determined will be applied gradually, rather than instantaneously.
Listen to the mix of your song again. If the volume of the vocal doesn't seem loud enough, reduce the threshold of the compressor. If the vocal track does not feel sufficiently under control, increase the ratio.
If you are unable to compress your track sufficiently at this stage, apply your limiter. By limiting the track, you are applying a very high ratio of compression at a very high threshold. Set the ratio of your limiter; this can range from 10:1 upwards, and aggressively prevents the sound wave from exceeding the 'limit' of the threshold.