There's no question defence is the hardest part of the game to get right. It helps if the lead gets you both off on the right foot. Rest assured no-one ever gets it right all the time. You either: (A) try to establish tricks for your side or (B) try not to give away tricks to declarer. There are many other styles of leading, but below is a quick run down of what are known as STANDARD LEADS
The standard rules for which card to lead are
a) top of a sequence or a broken sequence K Q J 10, K Q 10 9 (K)
b) top of an internal sequence K J 10 x x (J)
c) fourth highest from a good suit Q 10 7 5 2 (5)
d) lowest from three to an honour K 8 3 (3)
e) top of a doubleton 9 5 (9)
f ) MUD (middle, up, down) from three small cards, play the top card on the
next round 8 5 3 (5)
g) second highest from four small cards, play your original fourth highest card
on the next round 10 8 6 3 (8)
As in all contracts if your partner has bid a suit it will most often be right to lead that suit. However if partner hasn't bid then you must decide from a combination of your own hand and the opposition bidding which SUIT will either be the best attacking lead OR the safest lead. First you decide which suit to lead. Then you must lead the "right" card from that suit, so that partner has a chance to work out what to do if they get the lead later in the play.
There is one difference when leading against Suit or NT contracts
K Q 7 6 3
Against a suit contract lead the K but against a NT contract lead the 6. You want fast tricks in a suit contract, but slow tricks when defending NTs. When you lead top of a sequence against NT contracts you will normally have 3 cards in the sequence, broken or otherwise. See (a)
Sadly none of this helps you decide which actual suit to lead. All you can do is listen to the bidding and avoid leading their suits. Obviously you can't do this if they bid everything (!) If declarer on your right has bid a suit and in that suit you hold: K x x x then it's probably wrong to lead it, even if it's your only 4 card suit. If there's no obvious lead, then work backwards by deciding which suits you probably can't lead and then lead the suit left!
Be warned: nothing works all the time. That is why Bridge is the fascinating game it is. In everything you do there is an element of risk and chance. The idea is simply to do the best you can with cards you have and the information given to you by the bidding.