The best way to multiclass a fighter -cleric in DnD 5e, is to create a pure cleric and choose the war domain.
It really gives you everything you want mechanically. As a war cleric you gain access to heavy armor and martial weapons at level 1. You also gain bolts of inspiration which allow you to make an extra attack action a number of times equal to your wisdom modifier each day. At level 2, you gain a +10 to your attack roll using channel divinity, and at level 6 you can give this +10 to another player. At 8th level you gain 1d8 damage using spell slots on your attacks, and at level 14 this increases to 2d8 damage, then finally at level 17 you gain resistance to bludgeoning, piercing, and slashing non-magical damage. You also gain the spiritual weapon spell as a domain spell, which effectively gives another extra attack during battles in which you cast it. (Page 63 of the Players handbook)
However if you still want to multiclass, (to gain unique features from battlemaster, champions, or eldritch knights) you will need a 13+ in wisdom and a 13+ in strength or Dex before you are allowed to multi class. Be aware that your spell slots will effectively be cut in half (or half + 1/3 if you mutliclass into eldritch knight). Your spell slots are determined by the new spell slot table on page 166 of the players handbook, with your effective casting level being your cleric level + 1/3 of your Eldritch knight level.
Also, your ability increases are based on class level not character level. Depending on how you multiclass, you also will still need the war domain to get access to all weapon and armor proficiencies.(heavy armor) which proficiencies you get depends on which class you pick first.
In general, when mutliclassing, your new class only gives a limited number of proficiencies from that class as outlined in the table on page 164 of the Players handbook.
The only proficiencies you gain when you multiclass into fighter is: Light armor, medium armor, shields, simple weapons, martial weapons. (Page 164 of the Players Handbook)
The only proficiencies you gain when you multiclass into cleric is: Light armor, medium armor, shields. (Page 164 of the Players Handbook)
If you start as a cleric, you'll get prof bonus to wisdom & charisma saves, and access to light and medium armr, shields and simple weapons. (Page 45 of the Players Handbook)
If you start as a fighter, you'll get prof bonus to strength & constitution saves, and access to all armor, shields, simple weapons and martial weapons. (Page 45 of the Players Handbook)
When multiclassing you do not get the save proficiency from from the other class, and in the case of cleric and fighter you also don't gain any skills. (Page 164 of the Players Handbook)
However as a war domain cleric you will get proficiencies to heavy armor and martial weapons. (Page 63 of the Players Handbook)