I would not use For
myself. But will leave that for you to change it if you want. I would just use Table.
But this is small modification to your code
ClearAll["Global`*"]
A = l (t^u) (x)^3;
B = t^u (x + 2 l)^3;
C1 = t^u (x + 3 l)^3;
l = 3;
data = First@Last@Reap@For[x = 0, x <= 2, x = x + 1,
For[t = 0.01, t <= 0.03, t = t + 0.02,
Sow[{u = 0.25; x, " ", t, A, B, C1, u = 0.5; " ", A, " ", B " ",
C1}]
]
]
And now just do
TeXForm[data]

You can export the above and compile it using Latex to give

can you suggest how to export in scientific form or restricting the
data to fixed decimal places
I think it will best to do this at the source, i.e. when you generate the data. For example using Round[A,0.01]
will round the value to 2 decimal points only.
ClearAll["Global`*"]
A = l (t^u) (x)^3;
B = t^u (x + 2 l)^3;
C1 = t^u (x + 3 l)^3;
l = 3;
data = First@Last@Reap@For[x = 0, x <= 2, x = x + 1,
For[t = 0.01, t <= 0.03, t = t + 0.02,
Sow[{u = 0.25; x, t, Round[A, 0.01], Round[B, 0.01],
Round[C1, 0.01], u = 0.5; Round[A, 0.01], Round[B, 0.01],
Round[C1, 0.01]}]]]
Gives
TeXForm[data]
\left(
\begin{array}{cccccccc}
0 & 0.01 & 0. & 68.31 & 230.53 & 0. & 21.6 & 72.9 \\
0 & 0.03 & 0. & 89.89 & 303.39 & 0. & 37.41 & 126.27 \\
1 & 0.01 & 0.95 & 108.47 & 316.23 & 0.3 & 34.3 & 100. \\
1 & 0.03 & 1.25 & 142.75 & 416.18 & 0.52 & 59.41 & 173.21 \\
2 & 0.01 & 7.59 & 161.91 & 420.9 & 2.4 & 51.2 & 133.1 \\
2 & 0.03 & 9.99 & 213.08 & 553.93 & 4.16 & 88.68 & 230.54 \\
\end{array}
\right)
Compiling

There are many other Mathematica commands to Format numbers you can use also. You can also use NumberForm
like this
n=NumberForm[1.23432,{2,3}]
(* 1.200 *)
And then export n
to TeXForm. There is also AccountingForm
and others. Check help for different command to format numbers.