Choose the right wire
Stainless has a high chromium content. There are lots of varieties, the common ones being 304L, 316L and 430. The latter, often used in exhausts, needs to be pre-heated to 150-200°C prior to welding. Each type of steel will need a compatible welding wire. Use 308 wire if welding 304L to itself or to 316L. Use 316L wire when welding 316L to itself. Use 309L wire when welding 430 to itself, 304L or 316L – or when welding any of those to mild steel. Use argon shielding gas with 2.5% CO2 content.