Hi,
I usually do 1.d4, as my oppening, it helps me to take the Kingside's Bishop and then the Kf3 to do castles. I also try to keep a pawn in h3 to protect my Knight.
It is better if you can hold the Knght in f3 along with the f, g and h pawns to protect your king the Rook in f1 will help tp protect or to attack! Hope it helps!
If you play e4, in 95% of the cases you must castle, otherwise you will get under attack. You do not lose tempo, you win tempo in one move getting your king into safe place and activating your rook. Move 10 could be too late, consider doing it earlier. Mostly White castles kingside, at the same time you should be flexible when you choose the side of castling, sometimes if your opponent already advanced his pawns, it's better to castle in the opposite direction. You can intentionally castle queenside to "attack with the opposite side pawns". You should be careful attacking with the pawns from "the side of castle", it will open your king. Look at a few standard openings and a few GM games, it will give you an idea.
The main reasons for castling:
Because of these things, most positions (especially most well-known openings starting e4) require that the King castle early, and usually King-side. As long as your King or f2/7 square aren't under attack, you can delay the castle. Just always be aware of a sudden latent attack against either the e-file (especially if your e-pawn is gone) or the f2/7 square, when castling may be too dangerous or too late.
You should also NOT castle into a pawn structure that is compromised if it can be avoided, as this does not provide much better King defense (the only pawn structure close to as safe as unmoved f-g-h pawns are the same pawns with a fiancchettoed Bishop).
I suppose what I'm saying is that there isn't a move by which you should have castled. It's more of a question of where the attacks are coming from. There are positions in the Guioco Piano, for example, where White castles on the 4th move (that is, the earliest possible time). Defenses for Black that sacrifice the center (e.g. some Indian types) will frequently have Black castle ASAP, then just try to hold on against the coming onslaught. There are positions in the Sicilian where Black doesn't EVER castle, as both wings have had Pawns capture toward the center, making it the safest place for the King to be. There are frequently positions where a castle is made to one side or the other not because of King safety as much as it moves the Rook onto an attacking square AND joins the two Rooks (an aggressive castle, if you will). It just depends on the position. Any hard and fast rule in chess is bound to thinking incorrectly in a significant portion of your games; that's why chess has so much appeal.
I am a beginner and find when to castle and which side to be confusing. I usually open with an e4 and target the 10th move (roughly) to castle. I feel as if I am not only losing tempo but it rarely benefits me against the aggressive players. Hence:
1) When to castle?
2) Is there a better side off of the opening e4?
3) Do I attack with the opposite side pawns or the side of castle?