I know I am a little late in this discussion but what the hell.
Like the old P Funk song says-
"everything is on the one!"
however it is good to learn how to mix with the bass, hihat, and the snare imo.
Practice beat matching in your monitors first.
Meaning do your scratch cue and beatmatching in the monitors first before starting to cue and beatmatch with your headphones. OR vice versa do it all in your headphones together.
IT sounds like crap but If you can master this it will make things easier for you in the long run.
If you are DJ'ing - Rock, Pop, New Wave, Afro Beat, Calypso, Soca, Bhangra, Salsa, Afro-Cuban, Favela, Ragga, Dub, Cumbia, Reggateon, Bossa Nova, Samba, R&B, Funk, Soul, Disco, Hip Hop, Crunk, Eclectic electronic stuff, Dancehall, Hyphy and Reggae in any form its easiest to mark your records with the BPM. Especially if you dj these forms together. However I didnt start doing this until about a 2 months ago and I have been dj'ing for about 12 -13 years now. I can pretty much feel how many BPM's a piece of music is running at now but it helps if you need to draw for a record quick and you have a minimal amount of time to pitch and cut. Or with somethings its best to play from the edge and let them run out.
With D&B, Jungle, Dubstep, House, Techno and Electro- everything is in pretty much the same BPM range so u dont need to put the bpms on the records or sleeves- imo.
Also another good idea is to get doubles of the same record and just get a feel for how the pitch control works on your decks.
Some other tips- TRY TO NEVER MIX OFF OF THE SPEAKERS ON THE FLOOR IN THE CLUB OR BAR YOU ARE DJ'ING In - IF YOU CAN ALWAYS USE YOUR BOOTH MONITOR (unless they dont have a booth monitor).
Practice till, pitching, scratching and beatmatching is like breathing.
When I started practicing I would play records for shifts of up to 8 to 12 hours straight. I ate drank and lived dj'ing
Practice with others if possible. One of the things the old school heads would do with me is we would play 3 records each and then we would have to mix out of the last record- we would do this for hours. So no matter what the DJ played before you you had to mix out of it and quickly- and we all would play an eclectic set of records so there would always be surprises.
I was dj'ing out in about 5 months after I started.
Within a year I was opening for mix masters like Noel, Dj Abstract, UFO, Claude Young, DBX and Dj Godfather. Its funny at one of my first gigs for a D&B night back in 95 -I accidently picked up the needle off of one of the records that was playing on the floor speakers- lol The whole crowd just stared at me. I learned there - to try and make mistakes look like you meant to do them.
There are some other things that are good to know but they are a little more obscure and they are not necessary to know.