As always, it depends. If you're opening an ATM spread at a debit that's about half the spread width, that's basically a 50/50 bet on direction by expiration. OTM spreads are going to vary a bit more, and lean harder into direction - but they'll balance that by paying more for a smaller risk if you're right.
A call debit spread presumes a directional assumption (bullish) and a price at/above which you believe the stock will end up by a given date. As a general rule of thumb, you'll want to choose a short strike at or a bit below that price depending on the confidence you have in your prediction; the long you buy should be defined by your per-trade risk tolerance. It's not a bad idea to check out the expected move for the stock by the date you're considering; if your expectation is very different from the market's, you should make sure that you're that much smarter than everyone else.
For the most part, the greeks won't be especially influential, since your long and your short will mostly offset each other; however, there are still positions in which they'll work for you rather than against you. Debit spreads are typically short theta, which means that you'll be losing value on that spread every day - but sometimes, this can be minimized or even eliminated. Try different strikes for your long; you may end up neutral or even slightly long theta, which would eliminate that daily loss. Also, since a call debit spread is bullish and IV typically
decreases as the price of the underlying goes up, you'll want to be short, or at least neutral, vega.
If you
are short theta, then make sure you're far enough out in time that it doesn't hurt you much; theta begins to accelerate around 45DTE, so you'll want to open your spread at least that far out. Again, if you end up neutral or a little long theta, this is less of a concern.
For the rest of it, you'll just need to put on a whole lot of them and see how they work over time. There's nothing in trading that comes even close to being as valuable as real live experience.
