I wouldn't use a sleeping bag, I would get a quilt. Sleeping bags weigh more than quilts, while sleeping bags do have a back side it doesn't mean much in regards to insulation because you are compressing filling by laying on top of it making the fill on the bottom completely useless for insulation, instead your mat is your insulation. Also sleeping bags temp ratings are almost always rated for 15 degrees less than what is comfortable whereas quilts are spot on with their ratings, and with quilts you can have them custom built for whatever temp rating you want, and whether want a closed or open footbox. then add a bluff if needed for your neck and face, and a wool cap for your head and ears. It's better to get a quilt, or a sleeping bag, for colder than you expect because you never know what you might encounter in weather.
If buying a new quilt is out of the question due to finances then consider getting a warm liner to place inside your current bag, a liner will add about 10 degrees to your bags bottom end (true bottom end of adding 15 to that number then take the 10 off of the new number. There is a warmer liner than the standard rated one I just mentioned, it's made by Sea and Summit called the Thermolite Reactor Extreme, this liner will add 25 degrees to the actual true cold rating. You could use another sleeping bag as a liner, but that would add significantly to the weight and bulkiness of the carrying weight but add a lot to the cold rating, put the thinnest bag on the inside unless one is down and one is synthetic then the down bag goes inside.
Find a mat that has a good R rating, Thermarest makes a model, or use to make this model but it could be under a different name now, called the Mondoking this had an R rating of 11 which is as high of a R rating I've seen in a mat.
Always layer up and avoid cotton.
Don't camp out at the bottom of hills due to cold air can rush through valleys, and don't camp on top of hills due to being exposed to wind. If you have snow on the ground first walk where you want the tent to be to compress the snow down as far as you can get it because compact snow insulates better than loose snow. And make sure the door to the tent is perpendicular to the wind direction.
Warning: if you are an amateur camper with amateur gear camping at below the mid 40's could become dangerous, seriously take that into consideration, if you still insist on camping anyways I would suggest you first try it in your backyard at home so you will know a little about what to expect and what changes you need to make. Cold weather combined with rain can really make it much more dangerous at below the mid 40's. So always check the overnight weather forecast for where you will be camping to get a clue as to what you'll be looking at.
Never put your head inside the bag to stay warm, this will accumulate vapor from breathing and begin to make you colder and colder as the night goes on.
IF you are going to be experience high wind and blowing snow you need to get a 4 season tent to withstand the wind without tearing the fabric, and these aren't cheap especially the lighter backpacking models, if wind is not going to be a problem then a 3 season tent will be just as comfortable as a 4. Also in the cold condensation is your enemy, so you need a 2 layer tent, otherwise with a single layer tent that condensation will drip on you all night and again wet is your enemy in the cold. Smaller tents offer a bit more warmth than larger ones, so you have to balance the size out with what do you want inside the tent with you, a lot of single hikers/bike campers go with 1 1/2 person tent, I have a 2 person tent but I put all my gear inside the tent the with me, and in reality a 2 person tent is really only comfortable for 1 person and their gear.
You want to protect your clothes your not wearing from getting wet either by condensation or rain getting through your backpack or pannier by placing them into zip lock bags. I have all my clothes, food, any type of liquid, and fuel inside ziplock bags, this prevents a leak in anything liquid to spill out and damaging clothing and ruining food, damaging the inside of the panniers, same for the fuel, I then go beyond that and line my panniers with a cinch up plastic trash bag for extra precaution, even though my panniers are waterproof it's just extra insurance from outside water getting in and inside liquids from running the interior lining of the panniers.
Carry a couple of different ways to start a fire, I carry waterproof matches in a waterproof container called a stormproof match kit; and I carry magnesium flint rod that I can either use the back of my knife to strike to spark and catch kindling on fire, or I can scrap off a small pile of the magnesium and then strike the bar to make a spark and get the magnesium pile to burn. I also carry a small ziplock bag of dryer lint for kindling. Some people carry windproof lighters, and they're ok too, I just prefer more traditional fire starting methods, so whichever you choose is fine.
I would suggest you read as many websites and watch as many Youtube videos as you can about camping in the cold to get more thoughts on all of this, because cold camping can be deadly for the amateur so you really need to know what your doing and then test that knowledge in your backyard several times before you do an actual attempt.
This should be obvious but never cook inside your tent. Don't drink water before going to bed, and urinate before you go to bed, getting out of a warm sleeping bag into the cold to go pee is not comfortable at all.