Can you activate a planeswalker's loyalty ability during your first main phase before combat?

Planeswalker loyalty abilities are activated abilities that can be activated any time you have priority and the game is in a main phase with the stack empty — this is the definition of a 'sorcery speed' action (CR 116.2a, 402.5c). Your first main phase (Precombat Main Phase) qualifies perfectly.

CR 606.3 specifies that a loyalty ability may be activated only if no other loyalty ability of that permanent has been activated this turn. So you get exactly one loyalty ability activation per planeswalker per turn, but the first main phase is a fine time to use it.

There is no rule requiring you to wait until your second main phase or after combat. Many players choose the first main phase deliberately — for example, to use a +1 ability to build up loyalty before potential combat damage could threaten the planeswalker.

Example: You control Chandra, Torch of Defiance on your first main phase. The stack is empty and you have priority. You activate her +1 ability to exile the top card of your library. This is completely legal. You may then proceed to your combat phase normally.

Remember: once you've activated a loyalty ability of a given planeswalker this turn, you cannot activate another loyalty ability of that same planeswalker again this turn, regardless of which phase you're in (CR 606.3).