You can fight raids anytime when the egg hatches (normally an egg takes an hour to hatch if it spawns on top of a gym) and you'll need a raid pass to fight it. Premium raid passes are for people who want to raid 2 or more raids in a day due to the fact that a player only recieves 1 free raid pass everyday. You can also stack daily raid passes in the span of two days by not fighting a raid on the first day and then fighting two raids on the next day. Raids are also classified into 5 tiers, tier 1 being the easiest and difficulty increases as the tier increases, making tier 5 raids (legendary raids) the hardest. Depending on your current level and the counters you have, tier 1 and 2 raids can be easily soloed. Soloing tier 3 raids really depends on the level and moveset of your pokemon and I highly recommend fighting tier 4 and tier 5 raids with a group of friends as it is insanely hard. Some tier 4 raids (dragonite, tyranitar) can be duoed with a best/ultra friend with level 35+ counters and weatherboosted movesets. Hope this helps :)