How to engage two characters into love?

One suggestion is to have the character thinking to his/her own self that they'd never noticed how beautiful/handsome the other was. Not until this point anyway. Prior to that the character could have had friends that said, "Wow. Justin is H-O-T!" or "Daaayyyum, did you see the legs on Lacy?" Of course the MC would have been thinking, Huh? That sort of thing. That's a good way to get across to the reader that there's a certain attraction for the characters without letting it be known right away to the opposite Hero/Heroine.

The way they look at each other , the way they talk ( guys stumble some times) . the way she/he feels when the other talks about another boy/girl. Try those.

Feelings, of course. The MC is disappointed when the boy dates another girl. Envious when she sees them together. Develops an instant dislike for the other girl - she's not good enough for him!

There are tons of ways other than coming out and saying "I have a crush on Bob".

One of my favorite devices, because I have a cruel streak, is to have the one who isn't crushing say to the one who IS crushing, "You're such a good friend to me, you're like a brother. I'm so glad you never pressured me for romance." And have the crushing one force a smile while inwardly they crumble a little.

The character keeps thinking about the crush and s/he isn't sure why. It doesn't have to be "oh they're hot" type things. Just ordinary stuff that they can't stop thinking about and they haven't the earthliest idea why.