Don's tip regarding the stampsmarter website is sage advice.
Recommend you also consider purchasing an older edition of the Scott Standard Postage Stamp Catalogue via eBay
. Older editions are readily available and reasonably priced. Scott recently transitioned to a 12-volume set, which can be quite pricey. However, a new collector can pick up a volume at a time for a modest outlay.
Many opt to begin with Volume One which includes the United States and associated stamp-issuing entities.
Last point: Unless you're a philatelic investor, don't get hung up on catalogue pricing-valuation. Many collectors view catalogue pricing as placeholders. High quality, rare material often exceeds the published/listed catalogue price. Average and or common material, much lower.
First and foremost, enjoy.