Armor: Go with either Barbarian or Ancient Armor (For your first kill i'd suggest Ancient) Make sure the Armor is Maxed if it's Ancient
Equips: Ancient Axe ++, Hylian Shield (Or a VERY good shield, hylian takes like five mins to get if yer efficient) Multiple Bows (i say multiple due to the fact you can break em) You want for preference a multi shot bow or an Ancient, Any Arrow will do but Normal onces for Preference.
Food: You want a 30 min Attack lvl 3 food so 4 Bananas and a dragon horn will net you this.
Health wise: you want a fair few super foods that will boost your health all the way back to max so that if the lynel does kick yer ass you have a way of healing. 5 Durians will provide this and theres a handy spot right near Faron Tower.
Fairies: Did you know it's possible to own more than 3 fairies? Fairies will spawn at a fountain if you have 2 or fewer on you, so with this in mind get 4 fairies from a fountain (the one hidden in the gerudo skeleton is a good option) then once you have them Warp back to the shrine near the fountain and hold all four in your hand when you get back to the shrine, this will coerce more faires to spawn. ONLY get 3 for a total of 7 then warp back to the shrine this time hold 5 fairies on returning and more will spawn. So it's now possible to have 11 fairies, Combine this with mipha's Grace you have effectively 12 lives. Combine with daruks protection and you have 15 attempts on each fight.
This may not be well known but SAVE before you fight yer lynel victims, this way if it all goes pear shape you can just reload and try again without spending longer for set up
Strategy is simple: Aim for his face to stun him and then climb on his back and whack away, This method is probably the best for early on as your weapon won't take damage while attack on his back. If you are really good at bullet timing you can get more shots into the lynels head as you leap off.
Strategy 2: Learn to make good use of Urbosas fury and the charge attack with Ancient Axe, the combo of the two can drain lynels health relatively quickly when you run outta urbosas fury return to firing shots into his face.
Strategy 3: HIGHLY DANGEROUS: if you are good at glidebombing and have the stamina and potions to do so go for a flyby attack, make sure you've got a five shot bow and just rain unholy death of bomb arrows down.
Strategy 4: Learn Perfect Parrying, EVERYTHING can be parried including lynels. If you learn it well enough you can go literally hand to hand with em.
Finally Choose your lynel, For your first kill try the ploymus moutain lynel near zora's Domain as it's always the weakest. After you've learnt timings try for the hyrule castle pair as they are respectively a blue lynel and a white-maned lynel, or explore the world i know the baobab plains has two lynels that offer clubs (which are damn good weapons) and spears so might be worth it. Make sure you aren't fighitng a silver lynel right off the bat as that will hurt ALOT.
So you finally killed the biggest baddest beastie in hyrule......so what do you do now? MURDER THEM ALL, if you've been killing lynels sequentially you should be getting their gear and squishy bits, the lynel bits can go to upping the barbarian armor and the lynel gear is i'd personally say the undisputed best gear in the game. Aim for snagging Savage Crushers and Savage Bows (the five shot variant) with this and the barbarian armor you can take lynels down ridiculously quick.