I have 2 bikes;
my old Fila Taos rigid frame steel MTB that i’ve converted to a road/trail bike (i yoinked off the old Kenda semi-knobbies and put on a pair of WTB Slickasaurus slicks), the Fila is going to be my main road rig
a Trek 4500 mountain bike, i don’t ride really gnarly off road trails, i stick to mainly fire roads / rail-trails and roads, so i replaced the stock Bontrager Jones AC tires with a set of Serfas Vermin semi-knobbies, they have much less rolling resistance than the Jones skins
the roads in my area are usually in reasonably rough shape and not really road-bike freindly, i’d do a lot of swerving to avoid sections of broken pavement, wheras the wider tires on the Fila can handle minor broken pavement with no problems
i see a couple options for you here;
first, decide if you’re going to ride on the road, or off road more;
if you are planning to ride off road more, look at an inexpensive mountain bike (Trek 4300 or 4500), but put a set of semi-knobbies (like the Serfas Vermin) on it
if you plan on spending more time on the road, choose the cross bike (also called a comfort bike or a hybrid bike), it’s got a lighter frame and less agressive tires than a MTB, but the frame is sturdier and the tires are wider than a road bike
1; go to your LBS (Local Bike Shop) and look at some inexpensive mountain/cross bikes, ride them and see what bike fits you better, you’ll know when you find the right one, it’ll just feel right
2; go to a secondhand store/pawnshop/check the want-ads for a decent older bike, it may take some tuning up to get it back into rideable shape, but you’ll get a good reliable bike
3; get a wally-world bike, but be prepared to replace it in a year or two
the problem with #3 is that you won’t enjoy biking as much as you would on a better bike, a good, reliable bike that’s well maintained should last a minimum of 8-10 years, the initial investment will pay for itself over time
most of the common brands are decent, Trek, Raleigh, Gary Fisher, Cannondale, Giant, Felt, Fuji, Specialized, personally i like Trek, their frames have a lifetime warranty, and the components typically have a 1 year warranty, Giant and Fuji are decent too and are typically less expensive than the other “big” brands
i haven’t ridden one in well over 15 years, but Schwinn bikes are decent, if it was a choice between a “wally-world” special and a Schwinn, go with the Schwinn
here’s my Trek, mine’s a couple years older and doesn’t have disc brakes, just rim brakes, but they work just fine, discs can be a little “grabby” if you’re not used to them
my previous MTB was a Giant Boulder SE
if i had to buy a MTB right now, i’d either go with another Trek 4500, or a Raleigh M-80