1) do you mean using MFP's settings or do you mean doing TDEE calculations on another site? If you are doing the MFP way, adding your exercise cals in later, I'd set yourself to lightly active and eat your exercise cals. If you do them the TDEE - 20% on another site way, I'd calculate yourself as being in-between moderately and active.

I personally use moderately - also a desk job, probably something like 8000 steps per day (haven't worn my BMF in ages but I tended to do 6k on a low day and 11k on a high day so 8-9 would be average, and 6k was kind of a fluke).... ok maybe more like 9-10 since we moved and its a lot further to walk to lunch?

I don't do the spin classes though. So for you I'd do the calc for moderate, calc for average, and give yourself a point in between the two. At the very least, for you, I'd go to moderate. You're pretty active, it sounds like. And spin classes are hard work right? Never done one but I've heard they are big burns.

2) Its 'safe' to eat at a moderate deficit - you won't make quite as big of strength gains or muscle growth but that's ok. Its not a great idea to eat a HUGE deficit and do this but it won't really be dangerous, exactly, you'll just be weak and tired and miserable and sore all the time.

3) No, you don't need a belt. Eventually maybe for squats. But geared lifting is DIFFERENT - it does allow you to lift more but it also forces you to move differently, and it gives you support for weaker areas which is really important for competition lifters going for max weights, but not as important for someone that is trying to gain strength in general. You might one day want a belt, but in the meantime you do not need one and they help create artifical amounts of core strength, which is great for lifting more weight, but not great for building the core strength on your own.

Some quick google fu brings me this - which kind of sums up everything I've read -
http://articles.elitefts.com/training-articles/benefits-and-proper-use-of-weightlifting-belts/


And re: your macros. Those macros are PROBABLY ok. But there are downsides to taking the percentage approach. If you want to do a LOT of reading: http://www.bodyrecomposition.com/fat-loss/diet-percentages-part-1.html (and then part 2)

the TL;DR version is:
-- Get adequate protein (1.5g per kg of bodyweight - if you're used to lbs that is your weight in lbs divided by 2.2 and then multiply by 1.5)
-- Unless you're a body builder or an endurance athlete you can pretty much let the rest of it fall where it may as long as you eat a reasonably normal healthy diet - aka you eat food and aren't trying to live on protein shakes or weird processed stuff. Seriously, try it for a few weeks and see. Unless you are naturally very fat avoidant (you tend to eat a lot of sugars and a lot of stuff that says 'low fat' on the packaging) you'll probably find that your macros fall out somewhere in the 40/30/30 range naturally.

Just focus on whole foods, try to include fish, nuts, whole eggs, avocado in your diet for healthy fats (whichever of those things you like and that also fit into your calories well), and make sure you get adequate protein. The macros will probably work out AND you won't be stressed about them. This works best for me, honestly, other than that I kind of don't eat as much fish as I should because my husband hates it.

Also - expect some weight gain. Expect a fair bit of weight gain (and then be presently surprised if you don't). I am talking 5 lbs gain, MINIMUM. Probably not the first week unless the empty bar is really heavy for you but within a few weeks, you're going to put on glycogen and water weight like you wouldn't believe. Or just stay off the scale and take pictures.

I'm always excited for new people. When you are new to lifting you are going to see some amazing gains and physical changes.

Losing your balance on squats can be a flexibility issue (or well, a flexibility issue causing a form issue). Its probably not a weight issue.

How's your lower back? can you hold a good arch in your lower back (not a super huge one but your pelvis should be tilted downward).

Where's your weight distributed? Should be on your heels. If its not or if you find it difficult to squat that way, it could be hip, hamstring, calf, or ankle flexibility.

How close together are your heels and how turned out are your feet? Some women (and men but mostly women) need to set their feet apart a few inches wider than other people (mostly men again) because your hips are wider.

Stick with the empty bar again and try changing your stance around a bit. ALso make sure you've got the bar set up correctly - if you feel like you are HOLDING it with your hands/arm strength, its not set right. Ditto if you feel like its pushing on your spine. And an incorrectly set bar could also cause balance issues.