The frame of my bike is cracked where the seat post goes into the frame.

Hey, Local bike shop determined that both wheels were defective. I took the junk back to Walmart and got a refund... PROBLEM SOLVED!!

Check out https://gearly.org/ for more info.

Thanks.

There should be no cracking in the frame if the bike has not been dropped or crashed at some stage which has then resulted in the cracking appearing.
If the bike has not been abused and the warranty is still current and not expired then you can make a warranty claim if you have the proof of purchase .
Read the warranty terms and conditions first which should have come with the bike documents. If not available, the warranty detail is probably published on the manufacturer's web site.

The better bike manufacturers using very good quality frames normally offer a long warranty on the frame. That is not the case for all the cheapie bikes which most people tend to buy.

Tires size is on the tire. Frame size is measured from the center of the crank to the bottom of the seat post at the frame.

If I were you,and the frame is cracked...I would not ride that bicycle!

measure the diameter of the hole in the frame where the seat post goes. Get online and order one. They vary in size so get a precise measurement.

Web search it "remove seat stem from frame ". Many postings.

The chain guard is usually fastened to the seat tube of the bike (the frame tube coming up from the pedal area to the seat) with a plastic band. If this band is broken or missing, and there is excessive play in the chain guard, I suggest removing it entirely or returning the bike as it may be a hazard.

It could be one of several sizes. To determine the size you can measure the bike from the center of the bottom bracket (where the cranks install into the frame) up the seat tube to the top of the frame. This will give you the size of your bicycle.

I HAD THIS PROBLEM ONCE.SOMEONE PUT THE WRONG SIZE POST IN THERE...NOT ME.BE PREPARED TO RUIN THE POST TRYING TO GET IT OUT.I STUCK MINE IN A VICE AND SWIVELED THE BIKE ITSELF.PULLING AND TWISTING AT THE SAME TIME.TAKE AS MUCH OFF THE BIKE BEFORE YOU DO.THE POST WILL EITHER COME OUT OR IF IT DOESNT,TAKE IT TO A BIKE SHOP AND SEE IF THEY CAN HELP.GOOD LUCK

I'm going to assume you that when you say, "bike seat stem" you mean "seat post". I'm also going to assume that the bike frame is made of steel and the seat post is most likely chromed steel.

What most likely is the problem (absent of more details) is that the seat post has rusted to the inside of the frame. If the bike is stored outside this is a very common problem on less expensive bikes.

I have never heard of a bike manufacturer putting any kind of paint or other anti-corrosion inside of the bike frame to prevent rust.

You can try spraying a little "Liquid Wrench" or WD40 oil on the seat post where it inserts into the frame. Let it sit over night and then tap it with a mallet to try to break the rust free. Use a pair of Channel Lock pliers to get a bite on it and see if you can twist it out.

On a steel bike the very last resort to use, if you by all means have to have that seat post out, is to apply heat to it, with a propane or oxy-acetylene torch. This will probably destroy the paint job and weaken the seat post.

If the bike frame is aluminum, you can get something similar to galvanic corrosion and this can seize the seat post in the frame. The remedy is the same as with the steel frame (but excessive heat and easily melt aluminum very quickly and leave a puddle of molten metal on the garage floor). If the frame is carbon fiber and the seat post is aluminum, carbon and aluminum don't like to touch each other and they will start a natural chemical reaction of corrosion which could cause the two to stick together (don't ever use a flame on a carbon fiber bike, it would be the end of the bike and just plain stupid). Anytime aluminum touches carbon or titanium an anti-seize compound should be applied to the parts.

Once you have the seat post removed, use your finger to wipe grease inside the seat tube and on the seat post before reinserting it and you won't have this problem again.