I highly recommend the Sibley app for all birders. It allows you to compare two similar species side-by-side. You can view them in breeding plumage, not in breeding plumage, males vs. males, females vs. females, etc. It has a one-time cost of $20 which is definitely worth it considering that I use this app every time I go birding.
A common ID mistake among new birders is to assume a Townsend’s Warbler is a Yellow-Rumped Warbler
If you see either in flight, use this to try and ID it.
"Tails" of a New Birder's Journey 