Find a 12 inch ruler or a tape and measure chain elongation first. For me, a ruler works best. On a new mounted chain with the ruler held against a length of the chain, the ends of a 12 inch ruler should land on the same spot of 2 chain pins (center to center or edge to edge). The general rule is if the chain is elongated 1/16 inch beyond the 12 inches you should be able to just replace the chain. If the chain is elongated 1/8 inch you may need to replace rear cogs and chain rings. I've been able to get by with just replacing the chain at 1/8 inch on occasion so I always start with the chain first and evaluate how the rest perform with the new chain. The measurement should be relatively easy for you to do and will clarify the situation. There are also much better explanations online (Sheldon Brown, the almighty youtube, etc.).