The saddle may be your main problem, but also your fitness level. Don't rush to the local bike shop and buy an expensive saddle too quickly, as these can set you back a hundred bucks or more, just wait a little, build your strength and start learning proper biking technique and position. Your saddle in itself may or may not be the problem, so try to adjust its angle and height. Many newbies make the same mistake when they start biking cold turkey: their commute is too long, they don't have the proper technique and position, put all their weight on the butt, the bike components (saddle, handlebar, etc.) are not properly adjusted to the height and weight of the cyclist, and the bicycle has not been thoroughly maintained. Once you've checked your bike and did a good cleaning/maintenance, you can think about upgrading the tires immediately, but wait a bit to replace the other essential accessories for city commuting: a more comfortable saddle (forget about gel pads, they are useless), higher handlebars (a trekking handlebar is the best for commuting/touring). As a mustachian cyclist, I recommend to stay away from bike shops for maintenance or to buy parts and accessories. If your city has a community bike shop, that's the best place to look for cheap parts and help, and to learn about maintaining your bicycle yourself.